Generate plugin rst (#28901)

Generate rst docs for plugins 

Based on rst generated for modules. But generated plugin
docs go into docs/docsite/rst/plugins/$PLUGIN_TYPE/plugin_name.rst
( docs/docsite/rst/plugins/connection/ssh.py for ex)

* move plugins docs to rst/*_plugins/ subdirs for namespace
* Only gen support pages for modules for now.
* Add generated plugin docs to gitignore* add list_*_plugins templates
* support MODULES/PLUGINS filters for make htmldocs

   Add a 'PLUGINS=ssh' filter env var like MODULES to filter plugins to build docs for.

* fixup 'historical' version_added, skip plugins/loader.py
* Fix plugins_by_support ref link to new plugins/*/ location
* use :ref: for common_return_values, allow empty version_added
* warnings on missing doc info
* add a prefix to _random_choice
  It was colliding with the target for random_choice plugin

docs/docsite/Makefile.sphinx

@@ -2,7 +2,7 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    = -j $(CPUS)
+SPHINXOPTS    = -j $(CPUS) -n -w rst_warnings
 SPHINXBUILD   = sphinx-build
 SPHINXPROJ    = sdfsdf
 SOURCEDIR     = rst
@@ -17,4 +17,4 @@ help:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile.sphinx
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)