integrate coredev.buildout (was PR #1670)

[stevepiercy]

Preview of pull request

https://plone6--1671.org.readthedocs.build/contributing/core/index.html

Description

This time without submodules

ToDo before merge

• [ ] @jensens insert a Mermaid diagram in docs at https://github.com/plone/documentation/pull/1671/files#diff-36199f263c1ec44ec26fc136ea22e39344bf44acc065e03e0fe26bc263d2f9f2.
• [ ] Purge the files at the root in coredev.

ToDo after merge

• [ ] @plone/framework-team update links in to correct location https://plone.org/community/teams/framework
• Fix link in PLIP template http://docs.plone.org/develop/coredev/docs/plips.html to point to new docs.
  • [ ] Volto
  • [ ] Products.CMFPlone
  • [ ] Other?
• [ ] Update link in plips.md https://5.docs.plone.org/develop/testing/index.html

Closes

• Closes #1495
• Closes #1486
• Closes plone/buildout.coredev#840
• Closes plone/Products.CMFPlone#3973

References

• https://github.com/plone/documentation/issues/1278