Add in-doc labels for public facing features

[CosmicHorrorDev]

This tweaks the docs.rs setup, so that

• Docs are built with all features active
• The comrak::plugins::syntect module and ComrakExtensionOptions.shortcodes field are now labeled that they're activated by a feature

You can see the docs locally by running

RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features --open

Motivation: I had to go digging in the source code to figure out how to enable shortcodes since the current docs weren't built with that feature

[kivikakk]

Thank you!