Roadmap to PyStan 3

[riddell-stan]

3.0.0rc1 (around Feb 25)

• Finalize the user interfaces guidelines design document. (in particular, the draws-are-trailing-axis bit) (see User interface guidelines from stan wiki design-docs#25)
• Resolve all issues created before ca. 2020-12-01 in this repository and in httpstan.
• Replace the current pystan repository with this repository.
• Add a link to the old PyStan 2 documentation
• Announce pending deprecation on the Stan forum. Give ~1 month notice.
• Email/open issues in packages which depend on PyStan, ask them to pin to ~2. (Too much work.)
• Make rc1 release about a month after deprecation announcement.

3.0.0 (around March 25)

• Add FAQ section
• Write release notes
• Before release, remove all mention on "beta" from docs
• Mark PyStan 2 github repo as archived.

[riddell-stan]

I'll close this now. There's a PR with the last round of documentation updates here: #239