Add infrastructure for flight reports

[lkluft]

This PR:

• Adds a section for flight reports (including two showcase examples)
• Adds a Sphinx extension to automatically create an overview table of existing flights
• Adds several new roles, e.g. {front} which can be used to access variables defined in the front page.

There are still some open issues in this PR, although all functionality is working. However, the current branch is already quite extensive, so an intermediate merge seems appropriate.

[lkluft]

I pushed an additional commit trying to consolidate the various roles and scripts.

I am not sure if the individual commits really add anything to the clarity of this branch-we might consider squashing everything before merging back to main. For review purposes, I kept the history as is.

[yu71ng]

Thanks a lot for setting this up! It visualises a nice overview about operations.

Later on, we can think of adding a how-to or list important tags to use for the categories. But this can happen in a new PR. 😊