Infra: Move "Source" and "Last modified" from <article> to <footer>

[hugovk]

Looking at PRs like #4973, the Read the Docs comment says something like:

701 files changed · ± 701 modified

And lists all the PEPs. This is because they contain a "Last modified" timestamp in the footer.

For the RTD build, we're not fetching the whole commit history, so it's setting the last modified time as a recent time rather than the real last modified time. This doesn't really matter for the previews, better to have faster builds.

Right now, the PEP html structure is like:

<header></header>
<article>
  <section id="pep-content">/<section>
  <hr>
  <p>Source: ...</p>
  <p>Last modified: ...</p>
</article>
<nav id="pep-sidebar"></nav>

We could configure RTD to ignore the bit of HTML containing the timestamp, but RTD already ignores changes in a <footer>, and these things also semantically belong in a <footer>, which should also improve navigation for screen readers and so on.

Therefore this PR moves "Source" and "Last modified" to a <footer>:

<header></header>
<article>
  <section id="pep-content">/<section>
</article>
<nav id="pep-sidebar"></nav>
<footer>
  <p>Source: ...</p>
  <p>Last modified: ...</p>
</footer>

Also show the timestamp as "UTC" not "GMT": https://www.timeanddate.com/time/gmt-utc-time.html

[read-the-docs-community]

Documentation build overview

📚 pep-previews | 🛠️ Build #32757016 | 📁 Comparing 6c9bc21 against latest (78d2e5c)

  🔍 Preview build  

735 files changed · ± 735 modified

± Modified

• pep-0001/index.html
• pep-0002/index.html
• pep-0003/index.html
• pep-0004/index.html
• pep-0005/index.html
• pep-0006/index.html
• pep-0007/index.html
• pep-0008/index.html
• pep-0009/index.html
• pep-0010/index.html
• and 725 more...

[hugovk]

735 files changed · ± 735 modified

Hmmm... 🤣

[hugovk]

And of course, main still has timestamps in <article>, not <footer>, so we'll have to merge this first.