Improve docs for updating mainstream slugs

Lets keep this documentation closer to the codebase.

Original documentation has been updated to new usage of the higher level fabric tasks, however this rake task can document itself with a link to the main documentation for further info.
Merge pull request #289 from alphagov/feature/improve-docs-for-updating-mainstream-slugs

Improve docs for updating mainstream slugs
Update govuk_content_models version

govuk_content_models version 26.3.0 now returns the first edition
public_updated_at when no major changes have occured.