Add tagfiles to support fetching documentation with xeus-cpp-lite (#2862)

Hi,

We can currently fetch the standard documentation through xeus-cpp-lite.
But we can't yet cover any 3rd party lib.

1) Xeus-cling could fetch xtensor documentation (even without xtensor
being installed, because the tagfiles were added to xeus-cling
themselves. Check
[here](https://github.com/jupyter-xeus/xeus-cling/blob/main/share/xeus-cling/tagfiles/xtensor.tag))
. This doesn't sound justified.

2) As discussed with Sylvain in the past, we probably shouldn't populate
xeus-cpp with the tagfiles for all repos but rather have them be a part
of the repo itself. And then when we do a mamba install %xtensor, these
end up in the file system and then can be put to use from there.

Something like 

<img width="1528" height="418" alt="image"
src="https://github.com/user-attachments/assets/012d4c8b-f757-4120-821d-eca6a60a2c52"
/>

Author: anutosh491

.gitignore

@@ -63,3 +63,6 @@ __pycache__
 # Generated files
 *.pc
 .vscode/settings.json
+
+# Allow tag JSONs under etc/xeus-cpp
+!etc/xeus-cpp/tags.d/

CMakeLists.txt

@@ -12,6 +12,17 @@ project(xtensor CXX)
 
 set(XTENSOR_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include)
 
+# xtensor tag files
+set(XTENSOR_DATA_DIR "share/xeus-cpp")
+set(XTENSOR_TAGFILES_DIR ${CMAKE_CURRENT_SOURCE_DIR}/share/xeus-cpp/tagfiles)
+
+set(XTENSOR_CONF_DIR "etc/xeus-cpp")
+set(XTENSOR_TAGCONFS_DIR ${CMAKE_CURRENT_SOURCE_DIR}/etc/xeus-cpp/tags.d)
+
+file(COPY "${XTENSOR_TAGFILES_DIR}" DESTINATION "${CMAKE_CURRENT_BINARY_DIR}/share/xeus-cpp")
+file(COPY "${XTENSOR_TAGCONFS_DIR}" DESTINATION "${CMAKE_CURRENT_BINARY_DIR}/etc/xeus-cpp")
+
+
 # Versionning
 # ===========
 
@@ -290,6 +301,12 @@ configure_file(${PROJECT_NAME}.pc.in
 install(FILES "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
         DESTINATION "${CMAKE_INSTALL_DATADIR}/pkgconfig/")
 
+install(DIRECTORY ${XTENSOR_TAGFILES_DIR}
+        DESTINATION ${XTENSOR_DATA_DIR})
+
+install(DIRECTORY ${XTENSOR_TAGCONFS_DIR}
+        DESTINATION ${XTENSOR_CONF_DIR})
+
 # Write single include
 # ====================
 

docs/source/changelog.rst

@@ -26,7 +26,7 @@ Changelog
   `# 2848 https://github.com/xtensor-stack/xtensor/pull/2848`
 - Revive benchmarks
   `# 2854 https://github.com/xtensor-stack/xtensor/pull/2854`
-- Set cmake min version to represent used features. 
+- Set cmake min version to represent used features.
   `# 2852 https://github.com/xtensor-stack/xtensor/pull/2852`
 - Enabling rich display for xeus-cpp-lite
   `# 2853 https://github.com/xtensor-stack/xtensor/pull/2853`

etc/xeus-cpp/tags.d/xtensor.json

@@ -0,0 +1,4 @@
+{
+  "tagfile": "xtensor.tag",
+  "url": "https://xtensor.readthedocs.io/en/latest/"
+}