Fall back to system config location for non-root users if user config is not provided

[jhiemstrawisc]

I thought this was a feature previously, but it turns out it probably wasn't.

In any case, Pelican now falls back to its standard system config location '/etc/pelican' in the event that a non-root user doesn't supply their own config.

This behavior is needed for site-local caches because HTCondor jobs don't run as root, and system admins need a way to configure which Cache an EP will use without pre-generating config for each individual user of the EP.

[jhiemstrawisc]

Note that I decided to pull on a thread here a bit, thus the explosion in size -- most of the noise comes from the last commit, which is where I ran a markdown formatter on our docs to enforce the "one sentence per line" standard some subset of "we" agreed on some time ago but never enforced.

I will take this PR back out of draft mode once I double check that docs build/render correctly.

[turetske]

A few very minor comments. I didn't bother checking every change your markdown formatting did.

Obviously make sure the linter check succeeds and the docs look good and ping me again.

[jhiemstrawisc]

I gave up on trying to format .mdx files -- it turns out mdformat was breaking a lot of the JSX integration for our docs, so I had to remove it.

[turetske]

LGTM!