Make readme easier to read using <details> sections

[meoowe]

Reformatted the following sections to use GitHub's <details> for collapsible sections

• Rules for submitting a proposal
• What to do if your proposal was closed
• How core developers evaluate proposals

Also clarified what option to use when making a suggestion using a quick summary next to the link. The old discussion link would give a Sorry, we didn't recognize that category! Please choose one of the following valid categories. at the top of the page due to a missing /choose at the end.

[fire]

I am not convinced that hiding the Rules for submitting a proposal etc is best way to get people to follow the rules.

Mentioning people can use

for their reports might be a great tip though.

[fire]

Can you separate The [old discussion link](https://github.com/godotengine/godot-proposals/discussions/new) would give a Sorry, we didn't recognize that category! Please choose one of the following valid categories. at the top of the page due to a missing /choose at the end. into a different pull request?

[Calinou]

I'm not sure if we should use <details> for any of the sections, as it makes it less likely they'll be read. Hiding parts of the README in details tends to result in people glossing over it, especially since GitHub's CSS doesn't do a great job at styling <details> for better usability.