Use an rst file for galgebra_guide

This rst file was generated using nbsphinx and a very small script:
```python
from nbsphinx import markdown2rst
with open('galgebra.md', encoding='utf8') as f:
    md = f.read()

rst = markdown2rst(md)

with open('galgebra_guide.rst', 'w', encoding='utf8') as f:
    f.write(rst)
```
Since the md file is no longer used, it has been deleted.

The advantage of using plain rst is that we can more easily add cross-references, something that markdown is missing.
This should allow us to start to deduplicate the API docs.

The tables are ridiculously wide, but we can fix this manually without too much work, and the rendering is unaffected.

Author: eric-wieser

doc/Makefile

@@ -26,5 +26,4 @@ help:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	notedown galgebra.md > galgebra_guide.ipynb
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)

doc/README.md

@@ -12,11 +12,9 @@ The structure of the `doc` diretory is:
 │                                   #
 ├─ index.rst                        # The entry point of the Sphinx doc, it references both
 │                                   # galgebra_guide.ipynb and api.rst
-├─ galgebra.tex                     # The orignal LaTeX source that generated books/galgebra.pdf
-├─ galgebra.md                      # The Markdown source semi-auto converted from galgebra.tex
-│                                   # please edit this instead of galgebra.tex (deprecated by it)
-│                                   # or galgebra_guide.ipynb (auto generated from it)
-├─ galgebra_guide.ipynb             # The Jupyter notebook converted from galgebra.md
+├─ galgebra.tex                     # The original LaTeX source that generated books/galgebra.pdf
+├─ galgebra_guide.rst               # The convert vertion of the latex document.
+│                                   # This was converted to markdown manually, then to rST via nodedown and nbsphinx.
 ├─ api.rst                          # Use automodule to extract doc from galgebra Python source files
 │
 │                                   # Configurations: