doc/architecture: edit several sections

doc/architecture: edit several sections

Edit the following sections in doc/architecture.rst:

 1. Dynamic Cluster Management
 2. About Pools
 3. Mapping PGs to OSDs

The tone of "Dynamic Cluster Management" remains a bit too close to the
tone of marketing material, in my opinion, but I will return to firm it
up when I have finished a once-over of architecture.rst.

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>