addresses issue #372 by adding announcement popup & page

[finanalyst]

• announcements.rakudoc is the source for the anouncement page and is contained in 'structure-sources' for this PR
• When the website is opened an announcement modal pops-up
• Once dismissed, it never appears again for that announcement
• Adding a Note to the top of the announcements page changes the id of the last pop-up and so the new announcement will be popped when next the website is opened by a user
• announcements can be suppressed by toggle in the ABOUT dropdown.
• the SCSS is preliminary for both the pop-up and the page.

Changes to

• raku-styling to allow for popup HTML and More dropdown
• config/02-plugins to enable announcements
• added announcements plugin
• added test announcements.rakudoc file

[coke]

This is much nicer than the one off I was thinking of. Thanks!

[coke]

I did some testing, everything looks good (couldn't test suppression without pushing a new announcement.)

[finanalyst]

@coke repeating a couple of comments in IRC:

• ammend Website/structure-documents/announcements.rakudoc by putting another =Note :date<somestring> :caption<somestring>[nl]content at the top of the notes.
• commit to the docs-dev branch. (no need for authorisation, I think)
• it will take the CI about 15 minutes to run the tests (far too long)
• The CI needs to build the html so that the new javascript will be available on the docs-dev deployment

[finanalyst]

@coke now if there is no announcements page, there is no popup.