governance: Proposal Guidelines

[mochet]

If we look at all proposals submitted so far (https://joystreamstats.live/councils & https://testnet.joystream.org/#/proposals/historical) the formatting of the proposals varies quite significantly.

It would be good to have a document that lists out:

• Each type of proposal and their purpose
• Titling guidelines
• Guidelines for what information each proposal type should include (ex. "A spending proposal should include the current exchange rate")

There is no way to enforce these elements of proposal creation so they would simply be guidelines, but having more consistent proposals would help in many ways, especially so new users have a simple document that explains things.

It would also be good to link some good examples of each proposal type.