Revert "Remove preview content from structure branch."

This reverts commit 6b3b23bb5c610472e6a474f576d9801c5f0a120f.

Re-add this content to structure branch, as I am now adding the "guides" section.
Add raid1 setup guide under guides rather than posts.
Make generic_sorted much more generic.
Remove split changelog entries and the erb index collator.
Make one big document instead.
Reasons: simpler to maintain, less metadata to write, avoids needless recompiles of the items each run.
Remove split neo2-overlay entries and the erb index collator.
Make one big document instead.
Reasons: simpler to maintain, less metadata to write, avoids needless recompiles of the items each run.
Fix metadata for the RAID1 guide.
General Rules file cleanup.
Kill unused code, merge many rules thanks to changelog and overlay refactoring.
Create new neo2-overlay file, built of the previously separated files in the directory.