Generate reference docs from proto files

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, press enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug
/kind cleanup

/kind documentation

/kind feature

What this PR does / Why we need it:

Integrates gen-proto-docs into the project such that when generating Rust code from the protos, the reference documentation is generated at the same time.

Which issue(s) this PR fixes:

Closes #981

Special notes for your reviewer:

Hopefully this helps with #970

[markmandel]

Sorry for the long delay - got pulled into a bunch of other things, and it took me a while to wrap my head around how the xDS system was working. I believe this should be good now, and close out #970, and also should set us up to expand the docs more by adding comments to the proto file, since it powers a reference doc for that layer.

[quilkin-bot]

Build Failed 😭

Build Id: b1aba417-2142-497e-85d6-4548d09dee9f

Status: FAILURE

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[markmandel]

========================================================================
book/third-party/presentations.html
  request exceeded our ExternalTimeout --- book/third-party/presentations.html --> https://www.gdcvault.com/play/1029264/Open-Source-Game-Development-Summit
========================================================================

Interesting - if it continues to be a problem I'll ignore the link in the code.

[markmandel]

Weird, quilkin-bot didn't post. I'll have to look into why - but everything passed.

Edit: oooh, I know why - it loses the PR number when I hit "retry"