Improve handbook
While reading the Handbook, I've taken the opportunity to:
- fix several typos (also fixed in !1304 (merged) & !1323 (merged));
- port HTML headers to Markdown;
- add / update some links;
- fix a Markdown list broken by !1321 (merged).
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
While reading the Handbook, I've taken the opportunity to: