docs: add emoji symbols to improve docs

[jorgeorpinel]

Look at current notes, for example. Useful symbols:

• ⚠️ Warnings about possible problems related to DVC usage (similar to **Note!** and "Note that..." notes)
• 💡 Useful tips related to external tools/integrations

Some other emojis currently in use here and there: ⚡🙏🐛⭐❗

UPDATE: I just found also 📙🙂💎🏷📖 used in the change log (which may soon be removed though, see #897).

[jorgeorpinel]

I think we can close this issue and use it as future reference.

Maybe even move it to (or label it with) wiki?

[jorgeorpinel]

Actually, I put this in a new section of the docs contrib guide. See 5bf345b