[ui] add a docs ui route

[signebedi]

This is a little tough. I think we might need to save docs as an app config. That would mean setting a DOCUMENTATION_ENABLED bool and DOCUMENTATION_TEXT Markup string.

We can add a link with the title = "Read site documentation" and using the following bootstrap icon: https://icons.getbootstrap.com/icons/book/. Alternatively, we can write the Docs text with the Source | Privacy text at the bottom right of the footer.

[signebedi]

Should we handle this in markdown? I think it will save a lot of trouble. Cf. markdown would require an additional dependency, whereas we have everything we need for HTML right now.... But it's harder to write and lint than markdown, and we would probably need to install an html lint or something like that to write a validator route.

[signebedi]

[ui] [docs] find a way to handle images in docs
It's worth considering how to upload images for the site documentation.

[signebedi]

[api] [admin] admin route for docs management
We should create an admin route that will allow users to edit the contents of the docs page.