Speed up nox docs sessions (#13118)

* Use --jobs auto in docs nox sessions

On my 8 core system, a clean cold build takes 5-6 seconds instead of
10-11 seconds. Nothing major, but it's a welcomed QoL improvement. FYI,
this flag doesn't work and will be ignored on Windows.

* Stop installing pip twice in docs nox sessions

At some point, session.install("pip") in the docs and docs-live nox
sessions was changed to install pip in editable mode, presumably to
enable reruns w/o dependency installation (-R flag) to pick up changes
for our pip sphinx extension. This doesn't do anything though as pip
is reinstalled normally as it's declared in docs/requirements.txt.

I think it's a fair compromise that if you want to pick up changes in
pip's source that show up in the documentation, you should not be using
the -R nox flag.

noxfile.py

@@ -127,7 +127,6 @@ def test(session: nox.Session) -> None:
 
 @nox.session
 def docs(session: nox.Session) -> None:
-    session.install("-e", ".")
     session.install("-r", REQUIREMENTS["docs"])
 
     def get_sphinx_build_command(kind: str) -> List[str]:
@@ -143,6 +142,7 @@ def get_sphinx_build_command(kind: str) -> List[str]:
             "-c", "docs/html",  # see note above
             "-d", "docs/build/doctrees/" + kind,
             "-b", kind,
+            "--jobs", "auto",
             "docs/" + kind,
             "docs/build/" + kind,
         ]
@@ -154,7 +154,6 @@ def get_sphinx_build_command(kind: str) -> List[str]:
 
 @nox.session(name="docs-live")
 def docs_live(session: nox.Session) -> None:
-    session.install("-e", ".")
     session.install("-r", REQUIREMENTS["docs"], "sphinx-autobuild")
 
     session.run(
@@ -163,6 +162,7 @@ def docs_live(session: nox.Session) -> None:
         "-b=dirhtml",
         "docs/html",
         "docs/build/livehtml",
+        "--jobs=auto",
         *session.posargs,
     )