feat: add dismiss button to announcement banner

[jorisvandenbossche]

Addresses #1384

Using essentially the same approach as was used for the version warning banner in #1763

Some questions for the announcement banner compared to the versions:

• Do we also want to only remember it for 14 days, or just forever for a certain announcement?
• Ideally the stored dismiss state takes into account the content of the announcement, so that when you add a new announcement, all users see it?
• I am assuming that for a general announcement banner, we do not want this to be version specific (i.e. remember it separately per version)

Note: I only tested it with a remote announcement that gets injected, not yet with a local snippet.

[jorisvandenbossche]

This seems to work on the readthedocs preview as well: https://pydata-sphinx-theme--2287.org.readthedocs.build/en/2287/index.html

I am only not sure why the alignment of the close button seems to be different (left vs right) for the announcement vs version warning banner, as I don't see any difference in css

[Carreau]

Dismissing for 14 days is already a great step forward. I will try to review/merge that before end of month, but no promises.

[Carreau]

Ok, the month has been long; merging.