doc/start: Improve beginners-guide.rst

Make an unordered list to render correctly by adding a blank line.
Add seemingly missing words.
Capitalize consistently Monitor, Manager.
Don't claim that 3 Monitors are required for quorum. They are required
for redundancy. Reword the following sentences about quorum accordingly.
Try to improve language.
Use title case for section titles.
Trim spaces at end of lines.

Signed-off-by: Ville Ojamo <git2233+ceph@ojamo.eu>