[ADD] Upgrade documentation for developers

[hepi-nandoo]

More and more demand came from partners to have information about the upgrade process.
In this page, we intend to introduce the process we've been following at PS-tech for several year.
In the future there will be multiple pages around this one giving explanation on upgrade related topics.

[robodoo]

Pull request status dashboard.

[AntoineVDV]

Hi there, I see that the PR is still a work in progress (TODOs in the content, "Temporary PR" in the description, recent pushes...) but it was made ready for review; was it intended?

[hepi-nandoo]

Hello @AntoineVDV , we might have put it a few hours too early for review but it's ready now

[nmarotte]

Hey :D Quickly glanced over and noticed a few things that you might want to check :)

[AntoineVDV]

Hello, interesting doc 🙂

I put my review comments for the /developer part of the diff below. None of them are really blocking, so feel free to apply or discard.

Also,

• The previous rewrite of the upgrade documentation targeted 16.0. Wouldn't it make sense to target the same branch with this PR so that partners browsing the documentation in 16.0 can find the How-to guide too?
• I added a codeowner rule for the new file so the ci/codeowner_coverage should be green, but you must also fix whatever is making the ci/documentation_guidelines red.

[augustoPerez]

Hello @AntoineVDV

Thank you so much for your comments and inputs, I applied most of them (and set them as resolved).
I will continue with the remaining ones tomorrow.

As requested, the ci checks have been checked and all of them are green.

Regarding the taget version, this documentation is a general approach to Upgrade custom databases, it is independent from a specific Odoo version. The same applies to the previous PR: #6274
Does it make a difference the version that is targeted?
We chose 16.0 for the previous PR because Odoo 17 was not out yet.

[AntoineVDV]

In general, version-independent documentation PRs should target the lowest supported branch (15.0 here). Since the previous PR targeted 16.0, I recommend targeting 16.0 too and, in less than one year, 15.0 will be deprecated anyway. The reason is that users accessing the documentation from their Odoo database will be redirected to the version of the documentation that matches the version of their database.

[AntoineVDV]

Just squash the commits and we're good to go.

[augustoPerez]

@AntoineVDV commits squashed 😉

[AntoineVDV]

@robodoo r+