Improve the onboarding experience via matrix.org

https://blog.koehntopp.info/2024/02/13/the-matrix-trashfire.html gives the perspective of a person trying Matrix for the first time, and we can do much better!

The main gripes on the matrix.org website are:

- People on macOS miss the install link if they don't open the client details, and can go straight to the app store
- The landing page is too empty and doesn't say much
- The menu goes in responsive mode too early
- The Try Matrix section is too confusing for the general public, and leads to dangerous instances (improved in #2179 but there is still room for improvement)

The rest seems to be related to Element and Element X themselves and is beyond what the matrix.org website can do.

Review the current matrix.org landing page, client details and install links, responsive menu behavior, and Try Matrix section. Compare each area with the four user concerns in the issue and define the updated onboarding flow; done means these website-level problems are addressed without changing Element or Element X.

Written by the indexing model from the issue text.