Rework Gateway API, HTTPRoute, and GRPCRoute docs

[wmorgan]

Here's my attempt at reflecting the modern world of Linkerd. (Note: these are all in the 2.17/ subfolder; once we're good on content we should also just copy these over to 2-edge.)

Summary:

• Remove the top-level HTTPRoute feature doc. Replace it with a "Gateway API" feature doc
  • This doc includes instructions for how to have another project own the GW API on your cluster
  • This doc includes the version compatibility matrix
• Remove all language suggesting that users use the policy.linkerd.io types, and instead note that they are supported but deprecated
• Ensure all ServiceProfile docs (feature, ref, and guide) have a note that GW API types are now preferred, at the top
• Add a new GRPCRoute reference doc
• Make the HTTPRoute and (new) GRPCRoute reference docs point to the corresponding official docs for these types, and describe these docs as supplementary

Please review for technical accuracy as well as completeness. Note that the goal is to provide a practical guide to using the GW API types with Linkerd, not a comprehensive treatment of the GW API. Feedback welcome

[travisbeckham]

The changes look good. Now that markdownlint is passing, htmltest is showing some failed links. There are several places in the docs that link tofeatures/httproute/ which was removed in this PR. Do these need to be updated tofeatures/gateway-api/?

[wmorgan]

I fixed the broken links. In doing that I also cleaned up those docs, which expanded the scope of this PR quite a bit. We had a couple "tasks" docs that were very low-effort, which I removed entirely and put the content in the feature doc. I cleaned up a few other stylistic things like excessive hyperlinking.

[kflynn]

I like this so far, many thanks!! 🙂

[travisbeckham]

Looks good, although your latest cleanup resulted in more broken links.

[wmorgan]

Ok, I think I have finally made the linters happy. @travisbeckham should I also copy these changes over to /2-edge/ ? I think they are also applicable to 2.16, should I copy them to that subtree as well?

[travisbeckham]

Ok, I think I have finally made the linters happy. @travisbeckham should I also copy these changes over to /2-edge/ ? I think they are also applicable to 2.16, should I copy them to that subtree as well?

Yep, definitely copy all of these changes over to edge. And same for 2.16 if they apply.