Remove manual slug from Section schema

This information is already included in the URL they PUTed to, so it seems
unnecessary to require it. We still need to send it to Publishing API/Content
Store so that a section document knows which manual it is a part of, without
having to know about the structure of its base_path.
Slugs in request URLs must be lowercase

The GOV.UK slugs/URLs will be lowercase, so let's enforce it to avoid a
situation where they send it uppercase but we neglect to downcase it.

We will still need to remember to downcase Section IDs in breadcrumbs and
child_section_groups before we make them into slugs as part of base_paths.
Ensure that base_paths are always lowercase

GOV.UK URLs are case sensitive and the styleguide says they must be lowercase:
https://insidegovuk.blog.gov.uk/url-standards-for-gov-uk/
Add rules to README
Slugs in specs should conform to the styleguide

Practice what you preach.