Add back docs for Mautic 3 upgrade

[RCheesley]

We don't have the docs any more for the Mautic 3 upgrade steps which some users with very old instances might need.

Here's the docs: https://github.com/mautic/mautic-documentation/tree/main/pages/04.Mautic%203%20upgrade

Ideally we should have popped these on a Mautic 3 branch but we only started these docs from Mautic 4 onwards.

Suggestion: Branch from 4.x to create a 3.x branch (it will be quite similar) and then add the docs for the upgrade and create the redirect.

Note: URLS to redirect:

https://docs.mautic.org/en/mautic-3-upgrade/getting-started
https://docs.mautic.org/en/mautic-3-upgrade/upgrade-steps

[Joseph2001-braganza]

Hi @RCheesley so from what I have understood is to create a branch from 4.x to 3.x and will the previously added documentation of the mautic-3 will be replaced or will it persist and I have to add just two docs ?, wanted some clarity on this.
Thanks

[RCheesley]

Hi @Joseph2001-braganza - I think actually that it might make more sense to leave it in the 4.x docs branch, because there were a lot of changes from 3->4 so it might well be confusing for people coming to the 3.x docs and seeing things which aren't there in 3.

So I think I would suggest having a section in the 'How to update Mautic' page which explains how to upgrade from Mautic 2 -> Mautic 3 -> Mautic 4.

Some docs which might be helpful:

https://joeykeller.com/video-tutorial-upgrade-from-mautic-2-to-3/
https://joeykeller.com/update-mautic-3-to-mautic-4-via-ssh/

cc @jos0405 could you work with @Joseph2001-braganza and review this PR when it comes? Thanks!

[Joseph2001-braganza]

one last doubt
should we make the section for update matic from 2 to 3 in 3.x branch and the 3 to 4 in 4.x branch
or all together should be clubbed in 4.x branch like from 2 to 3 and then 3 to 4.

[RCheesley]

I think that at this time, given we don't have any docs for 3.x, we should do the 2 -> 3 -> 4 in the 4.x docs, and the 4 -> 5 in the 5.x docs. The 5.x docs can link back to the 4.x docs for how to update from earlier versions.

Then going forward as part of the major release process, we fork the 6.x branch and write docs on how to update from 5.x to 6.x in the 6.x branch and link back to the 4.x branch for earlier releases.

Does that sound like it would make sense as a user?

[Joseph2001-braganza]

Yeah pretty much .

[Joseph2001-braganza]

Hello @jos0405 @RCheesley I have opened the PR for updating 2.x to 3.x just check it if it is all good then I can proceed further adding the 3.x to 4.x documentation.