feat: add release workflow

[ericswanson-dfinity]

Description

Adds the release process workflow and documentation.

Notes:

• prepare-release.toml stores the settings for changelog rolling so that the later cargo release --execute step does not also roll them
  • One of the places cargo release looks for configuration is $WORKSPACE/release.toml, so this sibling seems reasonable.
• Commit 828e4ed referenced in the changelog is the first commit on the main branch

Fixes https://dfinity.atlassian.net/browse/SDK-1277

How Has This Been Tested?

I wasn't able to fork this repo (I imagine because it is private), so I made a different repo and then copied the changes here.

• PR #6: make 0.1.1-alpha.1 prerelease (no changelog rolling)
• PR #7: make 0.1.1 release (with changelog rolling)

I had to open the PRs manually, because gh pr create failed. It's possible that some repo setting is required, or that this is enabled at the org level.

pull request create failed: GraphQL: GitHub Actions is not permitted to create or approve pull requests (createPullRequest)