General provider deprecated support (without an init)

[thomasvl]

The html and markdown templates make use of providerInfo.init.deprecated.docString to support when a provider's initializer has a general deprecation comment; however the general provider doc string itself doesn't seem to support marking the whole provider as deprecated. It would be nice to see this added.

[tetromino]

Interesting point! And if we do that, probably we also should support deprecation annotation for rules, aspects, and symbolic macros.

Would require a change to the starlark_doc_extract proto output in Bazel - either to parse the deprecated stanza from the doc string, or to add it explicitly via provider(..., deprecated = "...") or similar.

[tetromino]

(To be clear, fixing this would require changes to both Bazel and Stardoc.)