docs: add URI scheme and QR code standards

[ATXMJ]

Add URI scheme and QR code standards, including specifications for the RFC-8905 payto: standard

[mistakia]

pushed some almost exclusively stylistic changes to your PR (https://github.com/mistakia/nano-community/pull/68/commits/75550567ba0b300e9f15554c2253305d52b90145)

quick summary:

• using code blocks instead of inline code styling for things that appear multiline
• change in header levels
• using headers instead of bold (allows linking to section and understanding section hierarchy)
• adding section spacers and table of contents
• removed italicization from italicized and capitalized text as it seemed harder to read imo. I think using one of: capitalization, bold, or italicization is enough to communicate emphasis. Not sure which is best but I think we should just pick one.

content changes

• replaced preferred with recommended
• added links to nano docs where appropriate

Let me know if there's anything that needs changing before a final review and merge. Will do a final review of the content for accuracy, otherwise it looks good enough to me.

[ATXMJ]

All changes look good to me!