Add guidelines for formatting questions and explanations

[knatten]

Suggestions from @tocic:

<changes> — <reason> — <automatable>:

1. use ``` instead of for code blocks (even inside quotes) — easier to read/write, code highlighting — greppable
2. no using namespace std; — more vertical space — clang-tidy/greppable
3. no EOL at EOF — consistency — clang-format can consistently do the opposite
4. 2 or 4 spaces for code indentation — consistency, more horizontal space — clang-format
5. (...) for omitting, [some text] for our remarks inside references — consistency — not
6. */** instead of _/__ — consistency, to support _ in refs — not
7. https instead of http — security — greppable
8. strip whitespace — easier to read — git
9. remove/replace non-printable chars — looks like they are present in the original standard draft so we can ignore them in the future — greppable
10. add missing punctuation — consistency, easier to read — partially greppable
11. use > instead of "" for refs — easier to read, better highlighting — not
12. empty line before a quote — consistency, easier to read in long explanations — greppable
13. > instead of just > — consistency, easier to read — greppable
14. use > (right angle bracket + newline) for linebreaks in quotes — vanilla markdown syntax, idk why it renders correctly without them — not
15. try to preserve the original formatting in quotes — hightlighting, easier to match — not
16. always add a hint — if there's "No hint", the user loses his score for nothing — greppable
17. refer to ¶note-X and ¶general-example-Y directly where appropriate — no need to write [*Note X*: and — *end example*] — not