Contributing.md: Various additions and edits (#7309)

Contributing.md: Various additions and edits (#7309)

Add a new point to encourage discussion before coding. Move the 'ask
before significant coding' suggestion into this.
Edit and extend commit message guidelines. Refer to core dev commit
messages as good examples.
Remove 'short' and 'briefly' from the description guideline to try to
improve description quality.
Larger text for '### A pull-request is considered ..'.
Add a second roadmap link to celeron55's blog roadmap which is the 1st
part of the forum roadmap.
Add full stops to lists.
Consistent text size for '## Donations'.