Document hpack logger configuration

[Mirochill]

Summary

• document that hpack emits its verbose encoder, decoder, and table diagnostics through named loggers
• show how applications can keep global DEBUG logging while raising the hpack logger level
• add the logging configuration page to the docs toctree

Refs #273

Tests

• git diff --check
• git diff --check origin/master..HEAD
• git show --check --stat --oneline HEAD
• Not run locally

[Kriechi]

I think the best practice is to use the approach outlined in https://stackoverflow.com/a/35326281 instead of introducing a new log level in hpack itself.

[Mirochill]

Thanks, adjusted in that direction.

I removed the custom TRACE level and code/test changes, and rewrote this as a docs-only update showing how to configure the named hpack loggers independently from an application's root logger. The PR title/body now reflect that it documents the recommended logger configuration rather than changing hpack's log levels.

Static checks run:

• git diff --check
• git diff --check origin/master..HEAD
• git show --check --stat --oneline HEAD