Flyout: ability to disable "keep the page when switching version/language"

[humitos]

We added the feature to keep the page when switching version/language from the flyout. This feature was requested in #211 and it was also how the old flyout used to worked.

However, there may be cases where the project owner may not want this behavior and want to disable it. I think we could add a setting in the WebUI to disable this behavior in case it's wanted.

I found myself wanting this for test-builds project since all the "versions" of this project are unrelated between them and even each "version" may use a complete different doctool (e.g. Sphinx, Docusaurus or Pelican). Opening a post on the Pelican version will make all the links from the flyout to be invalid since none of those files exist on any of the other versions.

I know this may be an edge case, because of the way how we use the test-builds project. However, I wanted to open an issue here to expose this to other users and receive feedback to decide whether or not implement the ability to disable it.

[humitos]

We've implemented the same feature for notifications in #390, so we may want to consider this config there as well.