Generate installation instructions in release notes

[dgp1130]

Currently, release notes are manually typed, yet there is a significant amount of WORKSPACE boilerplate which should be included. There is also a SHA specific to each version which is difficult to compute in advance manually. @aspect_rules_js has a release process which manually tars the workspace, computes the hash, adds it as a GitHub asset, and then generates the release notes. We should do something similar.

Ideally this would also generate a changelog from commit messages, but that is really a tangential problem.

[dgp1130]

After a bit of trial and error, I was able to publish 0.0.29-releasetest6 successfully and copy-paste the install instructions verbatim to a new project and have it work successfully.

I forgot to tag the commits, but they are all in this range: https://github.com/dgp1130/rules_prerender/compare/3bcb6123c3c966142db11d9fb1aadfd82683a163..c8d1ea1d354ac3a81e7c50411109b349aa61defd