DOC: Fix capitalization among headings in documentation files (#32550) 
Files updated:

```
doc/source/development/contributing_docstring.rst
doc/source/development/developer.rst
doc/source/development/extending.rst
doc/source/development/maintaining.rst
doc/source/development/meeting.rst
```

@themien can you merge master

cc @datapythonista  Thanks @themien 