Add documentation for loading non-bundled base Paragon styles

[brian-smith-tcril]

In this PR comment #38 (comment) I describe the limitations of the current implementation of the Paragon plugin's style hosting (specifically that it relies on the base Paragon styles that are bundled with the MFEs)

We should add documentation for site operators that want to load base Paragon styles more efficiently

Documentation we can provide:

• How to use jsdelivr URLs
• How to host your own base Paragon styles in this plugin
  • Site operators will need to figure out what versions they need, but once they do they can create a directory structure that works with the $paragonVersion wildcard.
• Other possibilities? Probably just link to https://github.com/openedx/frontend-platform/blob/master/docs/how_tos/theming.md to show what else is possible

[brian-smith-tcril]

Copying from my slack message

I think adding a "Loading base Paragon styles" block to #39 would be perfect.

Something along the lines of:

Loading base Paragon styles

By default, this plugin loads the base Paragon styles that are bundled with each MFE. This leads to users downloading a copy of the same base Paragon CSS for each MFE they navigate to before the styles load. It is possible to instead use shared base Paragon styles to improve first load performance.

Ways to use shared base styles:

• jsdelivr
  • documentation on how to configure the plugin to use jsdelivr urls for base styles
  • note that this hits the external jsdelivr CDN, so if that's an issue for your installation it's not a good choice
• Hosting your own
  • documentation on how to configure the plugin to use base paragon styles placed in the paragon static files directory
  • note that using the $paragonVersion wildcard means you'll want to have each version MFEs request in that directory
  • note that you can configure the URL to only use 1 version, that it should be fine if you aren't using nonstandard MFEs because all MFEs in a release use the same major version of Paragon so using a newer minor version won't be breaking, but still worth verifying