feat: Allow additional tools for sphinx_docs

[castler]

Some Sphinx plugins require that other (not necessarily Python) tools are available.

One example is the plugin https://github.com/basejumpa/sphinxcontrib-umlet, which requires that UMLet is somehow within the sandbox.

• Adds tools arg to sphinx_docs to allow passing in arbitrary tools
  that are made available at runtime
• Performs location expansion on extra_opts, which allows passing the
  location of the tools onto sphinx.

[aignas]

Since sphinx is enabled in the next release Ithink this does not need an extrachngalog entry, but the docstring for the tools could include a short example with a particular plugin. Why the opts need expansion is still a little unclear to me. Is it that some plugins may expect the tools to be found in the PATH and that case would not be covered. To be honest, I am not super familiar with sphinx and these questions could be just my ignorance.

[castler]

. Why the opts need expansion is still a little unclear to me. Is it that some plugins may expect the tools to be found in the PATH and that case would not be covered.

Sure, something like this?

sphinx_docs(
    ...
    name = ...,
    srcs = ...,
    extra_opts = [
        "-Dsome_binary=$(execpath //:some_binary_target)",
    ],
    tools = [
        "//:some_binary_target",
    ],
)

[rickeylev]

Mostly lgtm, just need the extra_opts doc updated (I couldn't suggest an edit since it wasn't a modified line)

[castler]

Mostly lgtm, just need the extra_opts doc updated (I couldn't suggest an edit since it wasn't a modified line)

Thanks, applied requested changes.