Implementation of tagging using sphinx-tags

[rekomodo]

Added sphinx-tags to the build requirements.

Had to supress warnings from Jupyter Book's external-toc extension from using external toctrees due to sphinx-tags generating some for the tag pages.
The supression looks like

sphinx:
  config:
    suppress_warnings: ["etoc.toctree"]

from this issue and is present in the book's config.

Resolves #23 .

[rekomodo]

I think it looks great, we might want to change the section's title though.

[RemDelaporteMathurin]

I was able to compile locally but in RTD there seems to be a couple of warnings still.

Would it be possible to have the tags not above the title? Like here for instance?

Also, I see we can colour the tags instead of having them plain. It looks a bit sexier!