Deprecate usage of mdx_showable?

[vinisalazar]

Hi,

When following the README instructions to build the website locally, I run into this error:

ERROR   -  Config value 'markdown_extensions': Failed to load extension 'mdx_showable'.
             File "/Users/vwsalazar/Bio/MB/MelBioInf_docs/mkdocs_dev/lib/python3.10/site-packages/mdx_showable/__init__.py", line 59, in <module>
               from markdown.util import etree
           ImportError: cannot import name 'etree' from 'markdown.util' (/Users/vwsalazar/Bio/MB/MelBioInf_docs/mkdocs_dev/lib/python3.10/site-packages/markdown/util.py)

Aborted with a configuration error!

When removing mdx_showable from mkdocs.yml, the website builds without issues. The repository for this plugin hasn't had any commits in 7 years, so I suppose it is quite outdated.

Apparently there are three cases where it's being used: two in the RNA-seq w/ Tuxedo tutorial (L256 and L435) and one in the RNA-seq advanced tutorial (L515).

I proposed deprecating usage of this plugin in favour of a solution native (see squidfunk/mkdocs-material#341) , or at least friendlier, to mkdocs.

Best,
Vini