sphinxdocs: allow repo rules to document their environment variables

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:
:::

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

Description of how its used.
:::
```

In all cases, it xrefs to `.. envvar` directives

Start by tracing the existing `repo-rule`, `envvar`, and docgen directives in the Sphinx documentation system. Determine where directive options and cross-references are implemented, then verify that repository rule documentation can list environment variables and that each `uses-envvar` entry links to the corresponding `envvar` directive.

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