Add short plugin description to each plugin under title

[cshanahan1]

Adds a short one sentence description of each plugin in the side menu under the plugin title, that is visible before the plugin is opened. This PR also enables search based on text in the plugin description.

[Jenneh]

Looks great!

[Jenneh]

Remove the 'docs description' if it only repeats information from the title description. In the case where new information is included, keep the description but remove the repetitive part.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 7 lines in your changes missing coverage. Please review.

Project coverage is 88.80%. Comparing base (3b00695) to head (faa55c6).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
jdaviz/core/template_mixin.py	46.15%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3268      +/-   ##
==========================================
+ Coverage   88.76%   88.80%   +0.04%     
==========================================
  Files         125      125              
  Lines       18961    19009      +48     
==========================================
+ Hits        16830    16881      +51     
+ Misses       2131     2128       -3     

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[rosteen]

I left one minor suggestion but other than that, looks good.

[kecnry]

just had an idea (not necessary for this PR, but it might be super simple) - could these strings be included in the logic for plugin search at the top? See trayItemVisible in app.vue... might just be trayItem.label.toLowerCase().indexOf(tray_items_filter.toLowerCase()) !== -1 || trayItem.plugin_description.toLowerCase().indexOf(tray_items_filter.toLowerCase()) !== -1 (basically any substring match in the title or description)

[kecnry]

This looks really great and quite excited about the search ability! (Thought for the future: we could also have a list of keywords for each plugin which aren't displayed but would still match the search).

One question: Why do some plugins set self._plugin_description and others self.plugin_description? Is there a benefit to going through the setter or not in the inits - and can we stick with one or the other?

I'm also going to open a small PR to your branch with some styling tweaks to tighten the padding. Feel free to take it or leave it or modify further. We can also run this by Jenn at some point for future additional styling tweaks.