Changes necessary for 10.15

[mmattel]

These are the changes necessary to finalize the creation of the 10.15 branch.

• The 10.15 branch is already pushed and prepared and is included in the branch protection rules.

• When 10.15 (core) is finally out, the 10.13 branch can be archived, see step 3 in Create a New Version Branch

• Note, that the 10.15 branch in this repo is already created, but the latest pointer on the web will be set to it automatically when the tag in core is set. This means, that in the docs homepage, latest will point to 10.14 until the tag in core is set accordingly. When merging this PR, 10.13 will be dropped from the web.

• Note that this PR must be merged before the 10.15 tag in core is set to avoid a 404 for latest.

• Note before merging this PR, we should take care that 10.13 has all necessary changes merged.

@pako81 @jnweiger fyi

@mmattel @phil-davis Post merging this, we need to backport all relevant changes to 10.15

[mmattel]

Maybe you want to put this PR in draft mode until you are really ready to merge it.

This is for the repo only and necessary that we can properly manage the branches here. The 10.15 branch itself is already created, this PR manages the build process - if triggered when loading 10.15. Only when switching the full docs to 10.15, the changes become active 😄