Implement period_archives common context variable

[djramones]

Motivation

As discussed in #2773 (specifically this comment), a context variable that lists the generated period archives along with their URLs should be useful for those who'd like to have an index to those archives. Before this, users had to manually write links to period archives in content or templates, or even expect site visitors to explore editing URLs on their browser address bar.

Implementation

I moved code from ArticlesGenerator.generate_period_archives() to generate_context(). Initially, the new context variable only needed the list of period/period_num tuples and the corresponding URLs, but in order to retain the generate_period_archives() functionality, I decided to include the generation of save_as and filtered articles/dates collections.

Also, I decided it appropriate to set default patterns for time-period *_ARCHIVE_URL settings, so that anyone using the new context var just by setting *_ARCHIVE_SAVE_AS would get sensible URLs by default too.

I thought about providing a default template implementing the new variable (perhaps named period_archives_index.html), but this is probably more of a website- or theme-level detail, so I just put in a sample HTML snippet in the docs.

Pull Request Checklist

Resolves: #2773

• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
• Updated documentation for changed code

[offbyone]

This looks good.

Should you add an entry to the changelog for this, or will @justinmayer do that on release?

[boxydog]

I left a few nits, but I don't see anything blocking.

Since I am new to the codebase, I could be missing something.

[justinmayer]

@offbyone: Normally we handle change-log entries via individual RELEASE.md files that should be included with PRs, but since we are merging a bunch of PRs into a single Pelican 4.9 release, I will handle this particular change-log bundle manually. You can see my collection so far in: #3219

[justinmayer]

Many thanks to @djramones for the enhancement and to @boxydog, @offbyone, and @avaris for reviewing. 💯