Create links from old docs to new for migrated docs

[andrewnicols]

As part of the migration we need to:

• create a new template on the legacy docs pointing to the new doc
• update code to use this new template as we migrate docs
• mark the docs are read-only
• mark any docs which we do not plan to migrate as obsolete + read-only

[andrewnicols]

I have created a new template in the legacy docs: https://docs.moodle.org/dev/Template:Migrated

This should be included on any page which is transferred over by adding the following to the source:

{{Template:Migrated|newDocId=path/to/new/doc}}

For example:

{{Template:Migrated|newDocId=/docs/apis/access}}

This template currently has no visible content but will be modified when we hit critical mass to announce that:

• the document is frozen
• it has been migrated to the new site
• link to the new location

This template also adds a new category which we can then use to identify all migrated docs for the purposes of freezing them.

[andrewnicols]

Closing as this has been implmeneted in a range of other issues.