Display an edit link on documentation pages

[Ostrzyciel]

This needs some special handling:

• Stable releases of RB docs are not editable, so including a link wouldn't make sense.
• The dev release's docs are editable, but you should edit this in the main RB repo.
• Datasets and profiles should be edited in RDF – with datasets being in their respective repos.

I'm not sure if it makes sense to cover all these cases, but some of these – certainly.

There is some handling for edit/view buttons in mkdocs Material, but it wouldn't work at all for us: https://squidfunk.github.io/mkdocs-material/setup/adding-a-git-repository/?h=edit#code-actions

Maybe we need to write a custom mkdocs plugin?

[Ostrzyciel]

From a duplicate task: #80

By default mkdocs material has buttons for editing and viewing the source of each page, but this doesn't make sense when your documentation is mostly automatically generated from sources in ~20 repos. Still, these buttons are absolutely crucial to making RB open to contributions and scaling the project.

We need some kind of universal widget that would allow editing a page or changing the metadata. For each page, there MUST be such a link to "Edit content" and if it also contains metadata, then "Edit metadata". The link button should be brightly colored and big, and lead to GitHub's editor with the option to later submit a PR with these changes.

Implementing this won't be trivial, what comes to mind is (possibly) an mkdocs plugin that would have a fixed set of rules to construct the correct link for a given path the doc repository.

This is kind of relevant to https://github.com/RiverBench/RiverBench/issues/52, we could implement that issue's functionality here as well.

Update the documentation page on editing documentation – there's a TODO for this issue there.