Split product handbook home

Why is this change being made?

Implements proposal one from Product#396 (closed)

This will probably break some anchors that will need to be cleaned up in various places, but this is mitigated to some extent by the nice index at the top linking to the new pages. If this seems like a good step forward I'd recommend merging it quickly or it will get conflicts that will be tricky to resolve.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what