Add support for writing applications in Markdown

[philippjfr]

• Add ability to panel serve, pn.serve() and panel convert .md files
• Add tests
• Add docs

[MarcSkovMadsen]

If you have time please test that a panel app .md file renders nicely on GitHub.

Then you have an easy way of deploying documentation.

Please consider whether the python tag could/ should be supported. Maybe it's better than the Panel tag. The Python tag is recognized and will render nicely on GitHub and in .md based docs or blog posts.

IDEs like Pycharm or VS code might also be able to provide help inside the code section if the Python tag is used?

[philippjfr]

If you have time please test that a panel app .md file renders nicely on GitHub.

Works fine, see https://github.com/holoviz/panel/blob/5593ff79585ea4fc1ef6e29d1611fe677e950c5b/doc/how_to/display/examples/hello_world.md

Please consider whether the python tag could/ should be supported. Maybe it's better than the Panel tag. The Python tag is recognized and will render nicely on GitHub and in .md based docs or blog posts.

IDEs like Pycharm or VS code might also be able to provide help inside the code section if the Python tag is used?

These are good points and originally I had support for python tags but my concern was that it made it impossible to actually render Python code inline as Markdown.

[philippjfr]

Just discovered that you can use the file extension as a language tag, e.g. .py works. So I'm tempted to change it.

[codecov]

Codecov Report

Merging #4602 (fbd53fb) into main (50ba78a) will increase coverage by 0.34%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##             main    #4602      +/-   ##
==========================================
+ Coverage   83.38%   83.72%   +0.34%     
==========================================
  Files         258      260       +2     
  Lines       36944    37036      +92     
==========================================
+ Hits        30805    31010     +205     
+ Misses       6139     6026     -113     

Flag	Coverage Δ
ui-tests	39.64% <29.89%> (+0.25%)	⬆️
unitexamples-tests	73.83% <92.85%> (+0.23%)	⬆️

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

Impacted Files	Coverage Δ
panel/io/jupyter_executor.py	0.00% <0.00%> (ø)
panel/io/markdown.py	90.47% <90.47%> (ø)
panel/io/convert.py	73.30% <100.00%> (ø)
panel/io/server.py	75.31% <100.00%> (+0.54%)	⬆️
panel/template/__init__.py	100.00% <100.00%> (ø)
panel/tests/command/test_serve.py	89.65% <100.00%> (+1.08%)	⬆️
panel/tests/io/test_markdown.py	100.00% <100.00%> (ø)

... and 10 files with indirect coverage changes

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