Release CMS 6 alpha

[emteknetnz]

Acceptance criteria

• A alpha version of CMS 6 is released
• The release excludes modules that lost commercial support unless otherwise explicitly specified in the linked issue
• The changelog includes an automatically generated list of API changes, similar to CMS 5.0.0's changelog
• API docs include CMS 6
• A blog post is published which calls out the release and:
  • Reiterates the release timeline for CMS 6 (when the beta, RC, and stable releases will be)
  • Explains the purpose of the alpha and points out we're still making more breaking changes
  • Highlights the change to supported modules
  • Calls for community members to email us if they want to maintain modules we're dropping support for
  • Calls for community members to explicitly try out the release and open issues (with "CMS 6 REGRESSION" in the title) for new bugs and unexpected breaking changes
  • Highlights a few of the changes we expect could be fairly disruptive (e.g. DBField and form field validation, SiteTree form field scaffolding)
  • Points out if there are any known issues (e.g. Remove DBField casting when submitting via forms silverstripe-framework#11453)
  • Indicates that future alpha releases may happen for select modules in isolation, rather than a full alpha2 of all supported modules
    • in that event there would not be an alpha2 changelog to accompany those changes, but changes would be reflected in the "releases" section in GitHub for the affected repos
  • Points out that there is currently no upgrade tool, though we will considering looking into one if there is sufficient community demand for one.
• The normal comms we do for releases are done (slack messages, forum post, etc)

PRs

• ENH Pull CMS 6 docs from the new branches doc.silverstripe.org#316
• MNT Add 6.0 to list of branches to build docs site developer-docs#648
• MNT Include colymba in releases recipe-kitchen-sink#77
• DOC Note that API links are broken for now developer-docs#649

[GuySartorelli]

Release exists and is installable.
Sink has the wrong constraint for colymba/gridfield-bulk-editing-tools (has 5.x-dev but should have 5.0.0-alpha1) - @emteknetnz do we care about that? I'm on the fence. We could delete the tag for sink, commit a fix to the composer deps, and re-tag it before doing comms? Or we could just leave it pointing at the wrong place.
The beta onwards won't have this problem, I've got a linked PR above to fix it.

I figure nobody uses the sink other than us and we won't be referencing the alpha tag, so probably not worth resolving?

[GuySartorelli]

Docs will be deployed once the linked PRs have been merged.
Still gotta do comms - pending silverstripe/api.silverstripe.org#122 (because otherwise the links in the changelog will 404)

[emteknetnz]

Just leave the sink alpha alone, we only ever use x-dev branches

[GuySartorelli]

Sweet.
I'll leave this open as a reminder that comms aren't done yet - but otherwise this is complete.

[GuySartorelli]

Done! Released with comms. API docs site will take some more work and is being handled separately - we've added a banner to the changelog pointing out those links won't work in the meantime.