FindBoost: Refine .rst formatting

Move the '::' starting literal blocks to the end of the preceding
paragraph.  Convert the "Boost CMake" divider to a section header.
FindBoost: Clarify example static libs option (#13663)

State explicitly in the example that it will only find static libraries.
Merge topic 'doc-FindBoost-updates' into next

f53878b FindBoost: Clarify example static libs option (#13663)
a42a665 FindBoost: Refine .rst formatting