Informative runtime error if myst is not installed

[mwouts]

With this PR, the md:myst and md:pandoc are always included in the Jupytext formats, and an informative runtime
error will occur if the required dependencies, resp. myst-parser and pandoc, are not installed. (#556)

For instance, if I don't have myst-parser, jupytext README.md --to md:myst results into

[jupytext] Reading README.md in format md
[jupytext] Writing README.md in format md:myst (destination file replaced)
(...)
ImportError: The MyST Markdown format requires 'myst_parser>=0.8'. Install it with e.g. 'pip install jupytext[myst]'

A similar error message is displayed in Jupyter Notebook or Lab user if they opens a myst file without having installed myst.

I have also simplified the documentation of the --to option in jupytext --help to make md:myst easier to find:

  --to OUTPUT_FORMAT    The destination format: 'ipynb', 'markdown' or 'script', or a file
                        extension: 'md', 'Rmd', 'jl', 'py', 'R', ..., 'auto' (script extension
                        matching the notebook language), or a combination of an extension and a
                        format name, e.g. md:myst, md:pandoc, md:markdown or py:hydrogen,
                        py:nomarker, py:sphinx, py:percent, py:light. The default format for scripts
                        is the 'light' format, which uses few cell markers (none when possible).
                        Alternatively, a format compatible with many editors is the 'percent'
                        format, which uses '# %%' as cell markers. The main formats (markdown,
                        light, percent) preserve notebooks and text documents in a roundtrip. Use
                        the --test and and --test-strict commands to test the roundtrip on your
                        files. Read more about the available formats at
                        https://jupytext.readthedocs.io/en/latest/formats.html (default: None)

[codecov]

Codecov Report

Merging #584 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #584      +/-   ##
==========================================
- Coverage   99.00%   99.00%   -0.01%     
==========================================
  Files          90       90              
  Lines        8883     8933      +50     
==========================================
+ Hits         8795     8844      +49     
- Misses         88       89       +1     

Impacted Files	Coverage Δ
jupytext/cli.py	96.75% <100.00%> (ø)
jupytext/formats.py	96.60% <100.00%> (-0.29%)	⬇️
jupytext/myst.py	97.40% <100.00%> (+0.09%)	⬆️
jupytext/pandoc.py	96.55% <100.00%> (+0.63%)	⬆️
tests/test_ipynb_to_myst.py	100.00% <100.00%> (ø)
tests/test_read_simple_pandoc.py	100.00% <100.00%> (ø)
tests/utils.py	100.00% <100.00%> (ø)

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[parmentelat]

thanks a lot; this is going to make first class a lot less tedious ;-)