Remove old info from gitfs top-level docstring

The warning no longer applies since auth support was added for pygit2.

Also added a link to the gitfs walkthrough.
Grammar correction
Fix sphinx build error

There is no 'math' lexer in sphinx.
Fix misspelled code-block type
Add new fileserver features to helium release notes

Also made some formatting fixes
Merge pull request #16666 from terminalmage/release-notes-helium

Add new fileserver features to helium release notes