📚👌‼️: Improve documentation and usability

[chrisjsewell]

This PR completely restructures the documentation and makes a number of key usability improvements:

1. Simplified top-matter for text-based notebooks

The previous usage of a jupytext configuration will still work, e.g.

---
jupytext:
  text_representation:
    extension: .md
    format_name: myst
    format_version: '0.13'
    jupytext_version: 1.13.8
kernelspec:
  display_name: Python 3
  language: python
  name: python3
---

but now the following can also signify a text-based notebook:

---
file_format: mystnb
kernelspec:
  name: python3
---

Note display_name is no longer required to be specified, since it is added as the name, if missing.

2. The cell metadata key render is replaced with mystnb, to make it consistent with the notebook metadata key. For old notebooks using render, this will still be read and issue a deprecation warning

3. The mystnb-quickstart CLI command has been added, to create a template Sphinx + MyST-NB project

[codecov]

Codecov Report

Merging #405 (8360c65) into master (ba22f95) will increase coverage by 0.30%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
+ Coverage   80.13%   80.44%   +0.30%     
==========================================
  Files          20       21       +1     
  Lines        2059     2132      +73     
==========================================
+ Hits         1650     1715      +65     
- Misses        409      417       +8     

Flag	Coverage Δ
pytests	80.44% <85.71%> (+0.30%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
myst_nb/glue/directives.py	69.85% <0.00%> (ø)
myst_nb/glue/roles.py	56.47% <0.00%> (ø)
myst_nb/glue/utils.py	64.86% <0.00%> (ø)
myst_nb/docutils_.py	77.94% <66.66%> (+1.66%)	⬆️
myst_nb/core/config.py	77.50% <81.81%> (-2.28%)	⬇️
myst_nb/sphinx_.py	89.36% <83.33%> (-0.47%)	⬇️
myst_nb/core/render.py	83.28% <90.00%> (ø)
myst_nb/cli.py	92.45% <92.45%> (ø)
myst_nb/core/preprocess.py	88.63% <100.00%> (+0.54%)	⬆️
myst_nb/core/read.py	80.11% <100.00%> (+0.46%)	⬆️
... and 1 more

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