Explore auto-generating api_version docs

[drevell]

Context: #461

As an abc maintainer, I want to make sure that all supported api_versions are documented in README.md. We want to avoid a situation where we release an api_version but forget to document it.

Possible solution idea: create a simple Go program that runs during CI. This program will fail the CI process if it detects an undocumented, released api_version. The program will run these steps:

• Look at the list of releases in decode.go to find the set of released api_versions (the ones with unreleased==false).
• Look at README.md to find the table that documents each api_version.
• Make sure each api_version from step 1 has a table row from step 2, and the table row looks like real docs (the table shouldn't just say "unreleased" or "not released yet").