Fix sphinx 5.0 ignoring :std:doc: links by default

[rkdarst]

• To avoid confusing, Sphinx 5.0 avoids looking up documents in other projects with a role of :std:doc:. What this means that if you link to :std:doc:`glossary`, it might accidentally link to glossary in one of the external projects. The new idea is that you wouldn't usually want to link directly to an external page (this is not symbolic and could be rearranged at any time). Instead, link to the symbolic description, e.g. :rst:dir:`glossary`. The exactly function/directive/etc. is a stable link which is good to link to. (you could say this is the same reason we don't want to use raw HTML links)
• Change the few :std:doc: external links to the rst directives that we actually meant to link to.
• This change should work in older Sphinx just as well. This was probably the better option when I was first adding this, too.
• Review: merge if tests pass.

[rkdarst]

This should have worked, but less than 30 minutes before this, sphinx version 5.2.0.post2 was released, which appears to cause an error in sphinx_rtd_theme. That version ID appears to be allowed (https://peps.python.org/pep-0440/#public-version-identifiers), but sphinx_rtd_theme assumes only two . characters for splitting it. Random incompatibilities have happened before, I will go to sleep and see if it gets fixed, but it's not too hard to work around this.