[IMP] upgrade: overhaul upgrade doc

[nmarotte]

Overhauling of the documentation for Upgrade as part of the need for Partner documentation for upgrade (task 3171474 )

This keep most of the information intact, merging the FAQ into the documentation itself, improves the structure (one page), remove redundancies, ...

It is part of a 2-phases project where the second phase would be to add information about upgrade for developers (currently sitting in developer/reference/upgrade.rst but that will be moved to developer/howtos.html) to contain informations about migration scripts, upgrading custom code, etc ...

Related PR : #5774 #5953

[robodoo]

Pull request status dashboard.

[nmarotte]

Hello @xpl-odoo @AntoineVDV , this is the final PR for the first phase of the overhaul of the upgrade documentation.

@xpl-odoo Around 95% of the changes were already reviewed by you, and the rest of the changes can be viewed here (/!\ Only the administration/maintain/upgrade.rst is published, everything in developer/upgrade.rst is postponed, so no need to review it, as it is not part of this PR ) : https://github.com/odoo/documentation/pull/5953/files/89ad30ed5d3db3b5d9b3aebd7aca03e29e7d6a31..7233cb7a351d604345942331ec3d0264cad72cd4# (The SLA is simply copy pasted from what already existed before)

The other 5% are the various I changes I did on this PR, one to fix this

Another one to remove the note about "valid" subscriptions (https://github.com/odoo/documentation/pull/5953/files#diff-eb11d05d17bcc7b76a9453b1cdca38bc15380c0e3fbda7d6f2e68aa63f521dffR128)

and the last one to remove a trailing whitespace

Don't hesitate to let me know if you want to re-review this one last time together, or if it looks fine.

@AntoineVDV I'll be waiting for your explicit green light for merging this PR. If accepted before the OXP, I will merge it on Wednesday during the OXP, probably during the Keynote, if you think that is a good idea (I'd also like your feedback about when to publish it @augustoPerez and @odoo-pda , don't hesitate to let me know if you want it to be published at a specific time and date)

Thanks everyone for your help :)

[xpl-odoo]

Just a couple of comments and minor spelling issues ;)

[AntoineVDV]

Always happy to see the content tabs used in the doc! 🎉

LTGM from a tech POV, except for the redirect rules (see comment below).

If accepted before the OXP, I will merge it on Wednesday during the OXP, probably during the Keynote, if you think that is a good idea

I recommend you rather merge that ASAP today because you still need to forward-port it to 17.0, and I will create and merge the "publish 17.0" PR today, as the doc is built overnight. i.e., merge it the day before you want it published.

Also, is there a particular reason to merge this PR in 16.0 and not 15.0 (or even 14.0)? If not, you should create a new backport PR to put this current PR's content as-is in earlier versions so that the instructions are version-independent.

@robodoo delegate+

[AntoineVDV]

These pages should each be replaced by a redirect rule.

[nmarotte]

@AntoineVDV There is no particular reason for it to be pushed to 16.0 should I therefore retarget to 14.0 and pray for the least amount of conflict ? :D

I will work on the redirect now :D

[nmarotte]

robodoo r+