Move the plone.behavior readme into plone/documentation

[stevepiercy]

Merge the content of plone.behavior readme into https://6.docs.plone.org/backend/behaviors.html.

Replace the content of the readme with a link to https://6.docs.plone.org/backend/behaviors.html.

Originally posted by @jensens in #1298 (comment)

[stevepiercy]

@jensens would you please review the revised issue description? I think we already moved content from the readme, and forgot to replace its content with a link to the new docs, and close this issue. This issue was created before the overhaul last December 2022. Here's its history:

https://github.com/plone/documentation/commits/6-dev/docs/backend/behaviors.md