rst: single backticks now render correctly in both rst2html and github

[timotheecour]

single backticks were already supported by rst2html however they didn't render as monospace in github.

this PR allows using .. default-role:: code (which rst2html now can handle instead of choking on it) which is interpreted by rst parsers (including github) as changing how single backticks text is rendered, changing it to monospace so that it appears like double backticks text.

After this PR, there's no need anymore to use double backticks anywhere, this allows (in a future PR) to do a sweeping change and change all double backticks to single backticks, which is simpler, shorter, and more familiar for most people (familiar with markdown), so we end up with 1 way to do things instead of 2.

example

before PR
see https://github.com/nim-lang/Nim/blob/devel/doc/contributing.rst

after PR
see https://github.com/timotheecour/Nim/blob/pr_rst_singlebacktick/doc/contributing.rst

note

here's a snippet from contributing.rst that contained (before this PR) both single and double backticks. As you can see, before this PR the single backticks didn't render as monospace:

before PR

after PR

source:

Each test should be in a separate ``block:`` statement, such that
each has its own scope. Use boolean conditions and ``doAssert`` for the
testing by itself, don't rely on echo statements or similar; in particular, avoid
things like `echo "done"`. Don't use `unittest.suite` and `unittest.test`.

future work

• a single (or just a few) PR that changes all double to single backticks, and does nothing else

links

• https://docutils.sourceforge.io/docs/ref/rst/directives.html#setting-the-default-interpreted-text-role

[ringabout]

nice