Simplify the TWIM Guide

At the moment, the [TWIM Guide](https://matrix.org/twim-guide/) is a bit of a wall of text. It is nice to have an exhaustive list of things we want people to do, but it can be overwhelming for newcomers.

To keep TWIM welcoming and not discourage people from posting their updates, we could adopt a simpler "10 things to know" approach, that could look like the server rules in e.g. https://mastodon.social/about

I would suggest the following items

1. Follow the Matrix.org Code of Conduct.
2. Mention the bot `@this-week-in:matrix.org`, and then write what you want to share about your project.
3. Use markdown to write your entry. The bot expects markdown, and you'll make the editors' life much easier!
4. Start your titles with `### `. Your posts are embedded in a blog post page, and it makes everything more legible.
5. You can submit multiple entries, about the same project or different projects.

And then point to the rest of suggestions we give, below.

Start by locating the source for the TWIM Guide at https://matrix.org/twim-guide/ and review how its current suggestions are organized. Simplify the introductory guidance into the proposed “10 things to know” list, then point readers to the remaining suggestions. Done means the guide is less overwhelming while retaining access to the full advice.

Written by the indexing model from the issue text.