chore: add canonical links for older versions of keyman developer docs

[mcdurdin]

For example, https://help.keyman.com/developer/8.0/docs/reference_includecodes is currently higher in Google search results than https://help.keyman.com/developer/language/reference/includecodes

Adding link rel canonical to those documents should resolve this.

[ermshiperete]

See also #1693

[mcdurdin]

@Linsner do you want to take a look at this one also?