Merge topic 'fetchcontent-reduce-boilerplate'

a94355c7b7 FetchContent: Add new command FetchContent_MakeAvailable()
611d5274de Sphinx: Add limited support for nested variables in docs
1a07e1b47d FetchContent: Trivial doc corrections

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !2830