Add handbook pages for current language support, and "how to add support for a new language"

Created by: dadlerj

We don't have a clear definition internally of what it means to "support" a language. The ability to search the language? The ability to filter searches down to the language (lang:)? Syntax highlighting? Fuzzy code intelligence? Precise code intelligence?

We should document this! And potentially list which languages are currently supported at which levels.

Additionally, if we ever DO want to add "support" for a new language, multiple Sourcegraph Eng teams need to be involved:

@_sourcegraph/code-intel for code intelligence and creating and enabling the language extension by default
@_sourcegraph/search for lang: search suggestions
@_slimsag for syntax highlighting
Any more I'm missing here?

This is painful to coordinate, and there's no easy way to determine what level of support we provide. This leads to issues like https://github.com/sourcegraph/sourcegraph/issues/12003 and https://github.com/sourcegraph/sourcegraph/issues/10154 (which was closed, but we still don't have a lang: filter for it!)

Having a checklist for how to support a new language could be helpful here.

Not urgent, and cross-team, so tagging @christinaforney and @poojaj-tech for triage.