Add more blocks to simple base template

[frederik-elwert]

This change makes it easier to create new themes by inheriting from the simple theme. It allows customization of the whole body (while still making use of the theme’s head), or individual parts of the body like header, menu, or footer.

Pull Request Checklist

• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
• Updated documentation for changed code

[justinmayer]

Any feedback on this PR from @getpelican/reviewers?

[boxydog]

It is traditional to have blocks for header and footer, so that seems like a decent idea. It might also be traditional to separate them out into footer.html and header.html, though I wouldn't insist.

I don't see customization of the menu, although that is mentioned in the description of this PR.

My bigger questions:

1. Is this actually useful for someone? What is the use case?
2. If yes, then is it possible to write a test?