Adds optional banner hooks for docs and blog pages

[fekete-robert]

Makes it possible to optionally add banners to the top of the doc and blog pages, just above the content.

NOTE: I haven't updated the userguide yet, will do it if the users who reported the related issues think this PR is good enough to solve their problem.

Related issues: #1784 and #1773

To trigger the banner, you have to:

1. Create a file in your project called layouts/partials/hooks/banner.html
2. Add the HTML code you want in the banner to appear
3. Set the banner: yes parameter in the frontmatter of the page you want the banner on.

If you want to use multiple different banners in your docs, you can create multiple files like: layouts/partials/hooks/<custom-banner-filename>, and set banner: yes and banner_file: <custom-banner-filename> in the frontmatter to display the specified file as banner. Note that currently only one banner can be displayed on a page

[fekete-robert]

I've added a page to the userguide about this feature, so it's ready for review.

[sftim]

Optionally, explain explicitly that layouts/partials/hooks/banner.html can serve a banner that is defined at build time, or even an element that actually gets populated client side.

Optional because the PR already says anything's possible, just some people might not realise how broad this is.