[docs] Add a "Last updated" date to reference in docs (reference guides, recipes, tutorials)

Description

Related to #19222, an incorrect date of Feb 9, 2011 appears in Google SERP in the tutorial "Building a Theme".

Goal

To prevent issues like this from happening and to convey transparency on the maintenance of the docs, It may be worth to add a "Last updated" date for docs pages and make it dynamically pull from git.

[github-actions]

Hiya!

This issue has gone quiet. Spooky quiet. 👻

We get a lot of issues, so we currently close issues after 30 days of inactivity. It’s been at least 20 days since the last update here.
If we missed this issue or if you want to keep it open, please reply here. You can also add the label "not stale" to keep this issue open!
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[github-actions]

Hey again!

It’s been 30 days since anything happened on this issue, so our friendly neighborhood robot (that’s me!) is going to close it.
Please keep in mind that I’m only a robot, so if I’ve closed this issue in error, I’m HUMAN_EMOTION_SORRY. Please feel free to reopen this issue or create a new one if you need anything else.
As a friendly reminder: the best way to see this issue, or any other, fixed is to open a Pull Request. Check out gatsby.dev/contribute for more information about opening PRs, triaging issues, and contributing!

Thanks again for being part of the Gatsby community! 💪💜

[gillkyle]

Found this issue that seems related: #9785

This PR is also implementing similar functionality: #19391