[Trivial] Docs: Capitalize bullet points

Summary:
This is a backport of core PR11003.
Due to documentation divergence, it has been extended to the whole
doc/ directory (excluding archived release notes) as well as
contrib/.
https://github.com/bitcoin/bitcoin/pull/11003/files

Test Plan: Read the docs.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3401