sphinxdocs: allow repo rules to document their environment variables

[rickeylev]

Starting with Bazel 7.1, repository rules can use repository_ctx.getenv() to get environment variable values, and they no longer have to be listed in the environ attribute of the repo rule.

This makes them hidden from docgen.

A directive or option should be added so that repo rule docs can describe what environment variables they use.

:::{repo-rule}
:uses-envvars: <csv list of names>
:::

:::{uses-envvar} env-var-name

Description of how its used.
:::

In all cases, it xrefs to .. envvar directives