Last docs link fixes (#39391)

* should not need <>, but fails without

* adds anchor to keywords page, uses it on plugins pages

* fixes envvar link errors

* harmonize file name and ref name as python_3

* removes undefined-lable from ignore list

test/sanity/code-smell/docs-build.py

@@ -38,7 +38,6 @@ def main():
 
     ignore_codes = [
         'literal-block-lex-error',
-        'undefined-label',
         'reference-target-not-found',
         'not-in-toc-tree',
     ]