Provide alias for "content" in template engines as "content" and "config" are very similar

[jonbullock]

Once alias is available "content" prefix can be deprecated.

Suggested by @ge0ffrey via IRC.

[ge0ffrey]

Proposal A) Deprecate content in favor of page.

[ge0ffrey]

BTW, this can already happen in 2.x: create an alias for content (for example "page") but still support content. Replace "content" by "page" in all docs. No backwards incompatible impact. This allows for a grace period if you want to remove "content" alltogether in jbake 3.0 or jbake 4.0.