* Docs: split builtins to their own page from library
* review feedback
* addressed Hugo's feedback
* update the What's Next page
* one more wording tweak
* move builtins to their own directory. fix the reference name
* moved pages need to be noted in tools/removed-ids.txt
* add Python to the builtins reference title
* add sphinxext-rediraffe for the library->builtins split
* update check-html-ids to handle rediraffe redirects
* now we don't need (page missing) for the redirected pages
* update other references to moved pages
* A seealso from builtins to library
* make a nice section for rediraffe settings
* move the builtins note to the end, as a seealso
* address merwok's comments
* cache looking for ids in files
* simplify the intro paragraphs
(cherry picked from commit 59c4bddb17)
