Use sphinx-doc@1.8 from Homebrew instead of Sphinx and sphinx_rtd_theme from PyPI

[jamiesnape]

Closes #13529.

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This change is 

[jamiesnape]

@drake-jenkins-bot mac-mojave-unprovisioned-clang-bazel-experimental-release please

[jamiesnape]

 Traceback (most recent call last):
   File "doc/sphinx_build.py", line 9, in <module>
     from sphinx.cmd.build import main
 ModuleNotFoundError: No module named 'sphinx'

So we are not even using the executable found by @sphinx? That is unfortunate.

[jamiesnape]

@drake-jenkins-bot mac-mojave-unprovisioned-clang-bazel-experimental-release please

[jamiesnape]

+@jwnimmer-tri for feature review.

[jwnimmer-tri]

+@soonho-tri for user testing, please.

Reviewed 6 of 6 files at r1.
Reviewable status: LGTM missing from assignees jwnimmer-tri(platform),soonho-tri(platform), labeled "do not merge" (waiting on @soonho-tri)

[soonho-tri]

The following works on my macs:

./setup/mac/install_prereqs.sh
bazel run //doc:serve_sphinx

Now macsim has robotlocomotion/director/sphinx-doc@1.8 installed.

Reviewable status: LGTM missing from assignee jwnimmer-tri(platform), labeled "do not merge" (waiting on @soonho-tri)

[jwnimmer-tri]

feature.

+@ggould-tri for platform review per schedule, please.

Reviewable status: LGTM missing from assignee ggould-tri(platform), labeled "do not merge" (waiting on @ggould-tri and @soonho-tri)

[ggould-tri]

I hadn't seen that `additional_search_paths` trick used before. TIL.

Reviewed 6 of 6 files at r1.
Reviewable status: labeled "do not merge" (waiting on @soonho-tri)