add deprecation chapter

[MrTango]

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📚 Documentation preview 📚: https://plone6--1850.org.readthedocs.build/backend/deprecation.html

[sneridagh]

Do you all think that this insertion point in the TOC is the most appropiate place for this subject?

[stevepiercy]

It is not. See #1850 (review).

[stevepiercy]

@MrTango I'm awaiting changes per #1850 (review) before merging. Please let me know. Thank you!

[stevepiercy]

@MrTango should I close this PR as abandoned and won't fix? It needs help before it can be merged.

[ale-rt]

@MrTango should I close this PR as abandoned and won't fix? It needs help before it can be merged.

It would be a pity because it is a very nice write up :/

[stevepiercy]

It would be a pity because it is a very nice write up :/

Indeed. However, my review has not been addressed. I don't know why. No response from @MrTango implies this PR is not needed. If you care to push it forward, please be my guest.

[jensens]

@MrTango should I close this PR as abandoned and won't fix? It needs help before it can be merged.

I will take a look. I wrote this initially back in the days... so I would like to see it survive.

[jensens]

Ok, this should work. @stevepiercy what do you think?

[ale-rt]

Thank you very much to all of you :)
@stevepiercy do you mind having another look?