docs: generate config value reference (scikit-build#1052)

This would make it easier to cross-reference. I was debating going for
an auto-generated one and integrating with sphinx directly or going with
the `nox -t gen` files, and I think I prefer the latter because it makes
it more obvious to review the generated text in all the places, wdyt?

I also want to unlock usage of sphinx roles in it, but the readme
generation gets in the way. My opinion is that if we have this, we could
either move the readme part to the generated page, or truncate it to
only display the first summary sentence similar to how `click` does it.

Depends-on: scikit-build#1051
Closes scikit-build#999

---------

Signed-off-by: Cristian Le <git@lecris.dev>

Author: LecrisUT

README.md

@@ -144,7 +144,7 @@ as environment variables. `tool.scikit-build` is used in toml, `skbuild.` for
 below:
 
 <!-- [[[cog
-from scikit_build_core.settings.skbuild_docs import mk_skbuild_docs
+from scikit_build_core.settings.skbuild_docs_readme import mk_skbuild_docs
 
 print("\n```toml\n[tool.scikit-build]")
 print(mk_skbuild_docs())

docs/api/scikit_build_core.settings.rst

@@ -41,10 +41,18 @@ scikit\_build\_core.settings.json\_schema module
    :show-inheritance:
    :undoc-members:
 
-scikit\_build\_core.settings.skbuild\_docs module
--------------------------------------------------
+scikit\_build\_core.settings.skbuild\_docs\_readme module
+---------------------------------------------------------
+
+.. automodule:: scikit_build_core.settings.skbuild_docs_readme
+   :members:
+   :show-inheritance:
+   :undoc-members:
+
+scikit\_build\_core.settings.skbuild\_docs\_sphinx module
+---------------------------------------------------------
 
-.. automodule:: scikit_build_core.settings.skbuild_docs
+.. automodule:: scikit_build_core.settings.skbuild_docs_sphinx
    :members:
    :show-inheritance:
    :undoc-members:

docs/index.md

@@ -73,6 +73,7 @@ about/changelog
 
 api/scikit_build_core
 schema
+reference/configs
 ```
 
 ## Indices and tables