Add documentation on content migration/saving pages programmatically

[williln]

Docs request

When doing a content migration from regular Django models to new Wagtail models, I found very few resources. Saving a Page object programmatically, for example, is not currently documented and best practices are not made available. I touched on this in my Wagtail Space US presentation (link).

Proposal

• Add a section to Advanced Topics, possibly called Migrating to Wagtail
• Include:
  • Link to Page model field reference w/relevant explanations of how specific fields are used
  • Code snippet and example of ideal way to save a Page instance programmatically, including creating the relationship to the corresponding Index page if using
  • Explanation of how to use the save_revision method and what the difference is between a Page and a Revision in this context
  • Explanation of how to backdate content, if needed for migration
  • Explanation of ways to preserve URLs (forcing Page to use existing URL path or saving a Redirect)

Future enhancements

• Specific instructions for migrating from Django, Drupal, Wordpress
• Links/explanations of strategies for converting content, especially text content, to something Wagtail understands (leveraging StreamFields, for example).

I'm happy to take this on, but do the maintainers think this is a good idea?

[willbarton]

1101 is also relevant re: migrating.

[loicteixeira]

How to migrate to Wagtail is a recurrent question so it might indeed be a good idea to have some documentation.

However we have to be careful on the wording and stress that it is for creating pages programmatically to migrate to Wagtail, not within a Django migration (which is a completely different story).