Doc/change documentation default

[hannahlanzrath]

This PR aims to set the default of the documentation to the release branch 5.1.X instead of the master branch. Additionally, Pre-releases are added to the release section.

Open Questions:

• Do we want a dedicated pre-release section, or ist it okay to list the pre release version there?
• Do we still need chore/developer_guide listed in the Branch section, or is this obsolete?
• Since the update of the Version tags in Git, the Banner logic telling the user which version they are currently looking at does not seem to work anymore (probably since master has the same version tag as the latest release version?) Also whatever is the latest cadet version, is currently pulled from GIT and not a variable in Sphinx. I have updated the logic in page.html to reflect that we now have a stable release branch, master as a dev branch and a pre-release. This is a bit bloated and would require us to be consistant with naming of the branches and pre-releases in the future. Pls give feedback.

Otherwise, from my side this is tested and ready to merge.

Related Commits:

• https://github.com/cadet/CADET-Website/commit/05776c625723f279d66d388d9e0cf1b6e008286c (added possibility to build doc from branches)
• https://github.com/cadet/CADET-Website/commit/1cc555668a9a22a016ca4678b2b5b797a8a63da0 (made 5.1.X static reference site)

With the new feature I added to build doc from branches, I have already deployed this website version, if you want to have a look.

[hannahlanzrath]

fixes #543

[jbreue16]

Do we want a dedicated pre-release section, or ist it okay to list the pre release version there?

I'd strongly prefer a dedicated section, but I think It'd be fine to list them under releases if that'd require way more work.. which I dont think it will?

Do we still need chore/developer_guide listed in the Branch section, or is this obsolete?

I think the guide is mature enough now and we can remove this, thanks for the hint.

Since the update of the Version tags in Git, the Banner logic telling the user which version they are currently looking at does not seem to work anymore (probably since master has the same version tag as the latest release version?) Also whatever is the latest cadet version, is currently pulled from GIT and not a variable in Sphinx. I have updated the logic in page.html to reflect that we now have a stable release branch, master as a dev branch and a pre-release. This is a bit bloated and would require us to be consistant with naming of the branches and pre-releases in the future. Pls give feedback.

I think our (pre-)release naming will be consistent in the future, we are following an established semantic there. I think whats missing is still a hint when we are reading documentation of an older release? Maybe we dont need that since one has to explicitly select that anyways. Opinions @lieres ?