Explain apps to website visitors

[jbednar]

Added text for the top of https://panel.holoviz.org/gallery/index.html to explain how to use it. Also wanted to add similar text to the Component Gallery, but I can't actually see where the source code for that page is:

These pages demonstrate what each Panel component can do and how to use it. 
Click on each thumbnail to see the component used in a fully runnable web page 
in your browser thanks to [wasm/pyodide](https://panel.holoviz.org/how_to/wasm/standalone.html); 
just press the <span style="color:green">▶</span> button on the first code cell to run all the cells.

[codecov]

Codecov Report

Merging #4888 (fb6cf27) into main (fbafa73) will increase coverage by 0.18%.
The diff coverage is n/a.

❗ Current head fb6cf27 differs from pull request most recent head eb601f3. Consider uploading reports for the commit eb601f3 to get more accurate results

@@            Coverage Diff             @@
##             main    #4888      +/-   ##
==========================================
+ Coverage   83.23%   83.41%   +0.18%     
==========================================
  Files         270      270              
  Lines       38130    38130              
==========================================
+ Hits        31736    31807      +71     
+ Misses       6394     6323      -71     

Flag	Coverage Δ
ui-tests	40.87% <ø> (+1.06%)	⬆️
unitexamples-tests	73.36% <ø> (ø)

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

see 9 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[jbednar]

Note that instead of the explanation in this PR, it may be better to provide an explanation of WASM/pyodide at one central location in the docs, and then have each of the various points where someone might encounter such code link back to that one location. The main spots are the getting started section, which does have a short explanation at https://panel.holoviz.org/getting_started/build_app.html but isn't really linkable for this purpose, along with the app and component gallery index pages. The one central location should have an example that is immediately interactive, unlike e.g. the getting started one where the first cell isn't interactive with or without pyodide and thus the user can't really tell that anything happened.

[philippjfr]

Agreed on having a central location. For now this is good and helpful though.