Convert more docs source to markdown

[mosabua]

Description

Specifically connectors that are rarely changed and don't use include fragments in their documentation.

The PR also includes a narrower scope on the reserved keyword test requested by @electrum in a separate commit.

Output HTML changes are:

• Dropped table column percentage values (which didnt work anyway, tables look better now)
• Some wrapping in HTML source and inserted space characters instead of linefeed - no render change
• Some anchors used to be implicit and rendered as external links, are now explicitly set and therefore rendered as internal anchors (accumulo.md and googlesheets.md)

None of the changes have a negative impact on the rendered look of the docs.

Additional context and related issues

Follow up to a bunch of other PRs
#18430
#18429
#18391
#18375

and earlier ones

Release notes

(x) This is not user-visible or docs only and no release notes are required.