Add linting for "meta" documentation files (README.rst, etc.)

[TibsAtWork]

We don't apply Vale linting to our "meta" documentation files, specifically:

• CONTRIBUTING.rst
• README.rst
• REVIEWING.rst
• .github/vale/README.rst

I believe it would be a good idea to do so, as they are still part of the visible presence of this repository, even if only on GitHub.

Solution: add $(SOURCEDIR)/*.rst and $(SOURCEDIR)/.github/vale/README.rst to the targets for vale in the Makefile rule for spell. Note that this would mean we no longer need the explicit case for $(SOURCEDIR)/index.rst.

(It's quite likely Vale will pick up some errors in those files - those should be fixed in the same PR)

[lornajane]

Updated the title to make it clear what the issue is asking for, rather than what's not there. Hope this makes sense