docs: use stardoc proto output to generate markdown docs (#1629)

The template language Stardoc uses (Velocity) is niche and fairly
esoteric, and requires a lot of experimenting to understand how to
make it produce the desired output. In particular, it largely assumes
whitespace doesn't matter, which makes it a poor fit for generating
Markdown, where whitespace often does matter.

Instead, a small Python program is used to consume the binary proto
output of Stardoc, which converts it to Markdown. This also makes it
easier to customize the overall output and re-use code for the different
types of objects rendered.

The visible changes to the docs are:
  * Module extensions are now documented
  * Repository rules follow the style of the other generated docs
  * Fixed the rendering of pip_repository docs -- it had an h2
    section which broke the section grouping of the API objects.
  * Puts some padding between the border and content for text in
    params/attrs/fields listings.

Other notable changes:
  * Make RTD builds use bzlmod. This is necessary so that the pip
    extension can be documented. It loads
    `@pythons_hub//:interpreters.bzl`, but that repo is only created
    when bzlmod is enabled)

---------

Co-authored-by: Ignas Anikevicius <240938+aignas@users.noreply.github.com>

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 074a7b9..b8e778e 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -109,7 +109,7 @@
 # Install sphinx for doc generation.
 
 pip_parse(
-    name = "docs_deps",
+    name = "dev_pip",
     experimental_requirement_cycles = {
         "sphinx": [
             "sphinx",
@@ -126,7 +126,7 @@
     requirements_lock = "//docs/sphinx:requirements_linux.txt",
 )
 
-load("@docs_deps//:requirements.bzl", docs_install_deps = "install_deps")
+load("@dev_pip//:requirements.bzl", docs_install_deps = "install_deps")
 
 docs_install_deps()
 
@@ -140,3 +140,9 @@
         "https://files.pythonhosted.org/packages/50/67/3e966d99a07d60a21a21d7ec016e9e4c2642a86fea251ec68677daf71d4d/numpy-1.25.2-cp311-cp311-manylinux_2_17_aarch64.manylinux2014_aarch64.whl",
     ],
 )
+
+# rules_proto expects //external:python_headers to point at the python headers.
+bind(
+    name = "python_headers",
+    actual = "//python/cc:current_py_cc_headers",
+)