Add publish date on CI

[andys8]

Description

This PR is supposed to add the information about when the book was published to the hosted version on https://book.purescript.org.

Motivation

Because of the history of the book, I personally always had the feeling that I'm looking at an outdated version. Having some indicator might be helpful that one is indeed looking at an up-to-date copy of the (awesome!) book.

Technical Considerations

The current PR is super simple. It will add a single line. It is mostly untested on CI, btw. I have no right to execute the github action, so testing wasn't really possible.

Alternatives

There is also https://github.com/badboy/mdbook-last-changed. It will add a footer to every page and, by default, refer to commits. This might be helpful, but is not actually what I'd intend with this PR. Therefore, I went with the simpler command-line approach, where the result was more to my liking.

[Zelenya]

Awesome. Let's try it. Approving the run

[andys8]

CI Test

[andys8]

Update

Changed CI to append more information. I'm open to any text or structural changes. Currently it would look like this.

## Release

PureScript v0.15.4

Published on 2023-08-26

![image](https://github.com/purescript-contrib/purescript-book/assets/13085980/7e044e02-77d2-4181-aed9-

We could also go with a more textual description like "This book was last updated on x and the examples were tested with PureScript version y".

[Zelenya]

If there's no simple way of getting the PureScript version, maybe we can be vague about it? The goal is to tell people that the book isn't stale. Something like The book is kept up to date; see package.yaml (link) for the ...?

[andys8]

If there's no simple way of getting the PureScript version, maybe we can be vague about it?

Either vague or referring to the one the tests and CI are executed with.
953e017

[Zelenya]

Oh, ok, it's just added a 3s

[Zelenya]

Looks good to me, @milesfrain what do you think?

[andys8]

So, who's going to merge? 😉

[Zelenya]

Thank you, Andy! Works as expected.