Rename All Proposals to Include RCPs

[darnjo]

Currently we're using one file per distinct spec and handling new versions within the spec itself (in GitHub).

While we can permalink to these items, it's not the permalink we want since it doesn't pick up any new, non-breaking improvements to the spec since each reference is by specific commit or tag.

To address this, each item in the /proposals folder will include the RCP, even if it's a different variation of the same spec.

DD 1.7 becomes rcp-36-data-dictionary.md and 2.0, rcp-40-data-dictionary.md.

Currently we just have data-dictionary.md and reference commits/tags for the version.

Note: this will also require changes to other things that use these links, such as the certification system as well as public links.