Add stable cf-docs snippet tags

[danielporterda]

Summary

Adds stable CF_DOCS_SPLICE_SNIPPET_* RST comment tags around the Splice documentation ranges consumed by cf-docs external snippet generation. This lets cf-docs extract snippets by source tags instead of brittle line numbers.

Six implicit literal blocks are converted to explicit code-block directives so the tag comments can wrap the exact snippet body cleanly without changing the rendered intent. I also fixed one adjacent inline-literal spacing typo caught by sphinx-lint.

Confirmed that existing docs generation still works.

[isegall-da]

(see slack, I don't understand what we're trying to do. These snippets are no longer maintained in Splice, so why do we care about tagging them??)