gh-36911: src/doc/bootstrap: Parallelize generation of SPKG.rst files
    
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes #1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->
This makes running `bootstrap` a bit faster.

We also improve the RST markup a bit.

<!-- Why is this change required? What problem does it solve? -->
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

- [x] The title is concise, informative, and self-explanatory.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: #36911
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

build/bin/sage-spkg-info

@@ -5,11 +5,26 @@
 #
 #  Assumes SAGE_ROOT is set
 PKG_BASE=$1
+if [ -n "$OUTPUT_DIR" ]; then
+    exec > "$OUTPUT_DIR"/$PKG_BASE.rst
+fi
+if [ -n "$OUTPUT_RST" ]; then
+    echo ".. _spkg_$PKG_BASE:"
+    echo
+    ref ()   { echo ":ref:\`$1\`"; }
+    issue () { echo ":issue:\`$1\`"; }
+    code ()  { echo "\`\`$*\`\`"; }
+else
+    ref ()   { echo "$1"; }
+    issue () { echo "https://github.com/sagemath/sage/issues/$1"; }
+    code ()  { echo "$1"; }
+fi
 PKG_SCRIPTS="$SAGE_ROOT/build/pkgs/$PKG_BASE"
 for ext in rst txt; do
     SPKG_FILE="$PKG_SCRIPTS/SPKG.$ext"
     if [ -f "$SPKG_FILE" ]; then
-        sed "1,3s/^ *Sage: Open Source Mathematics Software:/$PKG_BASE:/" "$SPKG_FILE"
+        # for sphinx 4.4 we need to replace all direct links by some "extlink" (issue 33272)
+        sed -e "1,3s/^ *Sage: Open Source Mathematics Software:/$PKG_BASE:/" -e "s|https://github.com/sagemath/sage/issues/\([0-9]*\)|:issue:\`\1\`|g" -e "s|https://arxiv.org/abs/cs/\([0-9]*\)|:arxiv:\`cs/\1\`|g" "$SPKG_FILE"
         break
     fi
 done
@@ -39,9 +54,9 @@ for dep_file in dependencies dependencies_order_only; do
                 # Dependencies like $(BLAS)
                 \$\(*)      echo "- $dep";;
                 # Looks like a package
-                *)          if [ -n "$OUTPUT_RST" -a -r "$SAGE_ROOT/build/pkgs/$dep/SPKG.rst" ]; then
+                *)          if [ -r "$SAGE_ROOT/build/pkgs/$dep/SPKG.rst" ]; then
                                 # This RST label is set in src/doc/bootstrap
-                                echo "- :ref:\`spkg_$dep\`"
+                                echo "- $(ref spkg_$dep)"
                             else
                                 echo "- $dep"
                             fi;;
@@ -114,15 +129,15 @@ else
     if [ -f "${SPKG_CONFIGURE}" ]; then
         if grep -q SAGE_PYTHON_PACKAGE_CHECK "${SPKG_CONFIGURE}"; then
             echo "If the system package is installed and if the (experimental) option"
-            echo "--enable-system-site-packages is passed to ./configure, then ./configure"
+            echo "$(code --enable-system-site-packages) is passed to $(code ./configure), then $(code ./configure)"
             echo "will check if the system package can be used."
         else
-            echo "If the system package is installed, ./configure will check if it can be used."
+            echo "If the system package is installed, $(code ./configure) will check if it can be used."
         fi
     else
         echo "However, these system packages will not be used for building Sage"
-        echo "because spkg-configure.m4 has not been written for this package;"
-        echo "see https://github.com/sagemath/sage/issues/27330"
+        echo "because $(code spkg-configure.m4) has not been written for this package;"
+        echo "see $(issue 27330)"
     fi
 fi
 echo

src/doc/bootstrap

@@ -188,7 +188,7 @@ All External Packages
 EOF
 
 OUTPUT_INDEX="$OUTPUT_DIR"/index_alph.rst
-cat >> "$OUTPUT_INDEX" <<EOF
+(cat <<EOF
 
 Details of external packages
 ============================
@@ -202,16 +202,12 @@ Packages are in alphabetical order.
 
 EOF
 for PKG_BASE in $(sage-package list --has-file SPKG.rst); do
-            PKG_SCRIPTS=build/pkgs/$PKG_BASE
-            # Instead of just copying, we may want to call
-            # a version of sage-spkg-info to format extra information.
-            # for sphinx 4.4 we need to replace all direct links by some "extlink" (issue 33272)
-
-            (echo ".. _spkg_$PKG_BASE:" && echo && OUTPUT_RST=1 sage-spkg-info $PKG_BASE) | sed -e "s|https://github.com/sagemath/sage/issues/\([0-9]*\)|:issue:\`\1\`|g" -e "s|https://arxiv.org/abs/cs/\([0-9]*\)|:arxiv:\`cs/\1\`|g" > "$OUTPUT_DIR"/$PKG_BASE.rst
-            echo >> "$OUTPUT_INDEX" "   $PKG_BASE"
+    echo "   $PKG_BASE"
 done
-cat >> "$OUTPUT_INDEX" <<EOF
+cat <<EOF
 
 .. default-role::
 
 EOF
+) > "$OUTPUT_INDEX"
+sage-package list --has-file SPKG.rst | OUTPUT_DIR=$OUTPUT_DIR OUTPUT_RST=1 xargs -P 0 -n 1 sage-spkg-info