docs(config): document auth_type and other connection keys

[tobixen]

This is a draft PR as for now. Claude made something up, no QA done, no human reviews, and (while I may be wrong) I believe documentation written (or rewritten) by me is better than the AI-generated documentation.

Commit message:

docs(config): document auth_type and other keys

The config file page listed only six caldav_ keys and three CALDAV_
environment variables, so auth_type looked unsupported there even though
it has been a connection parameter since 3.0.  Both tables now carry it,
and a note after each says that any other CONNKEYS parameter works the
same way - with the two caveats a reader would otherwise hit: auth takes
an object and so cannot come from a file, and environment variables are
never type-converted, so CALDAV_REQUIRE_TLS=false sets the parameter to
the true string "false".

Prompt: is auth_type ('basic' vs 'digest' vs 'bearer') configurable today?  As a parameter into get_davclient and/or into the configuration file?
Followup-Prompt: What version did this go in?  Please update the doc [= the config file format documentation] in a separate work-branch and create a draft PR. (...)

Assisted-By: Claude Opus 5 <noreply@anthropic.com>

[tobixen]

This is a partial response to #713