[docs] Reword web app/landing page/mobile app sections after rearranging "Running keyserver" section

Summary: Depends on D4399. Relevant Linear issue with full context here. After moving the "Running keyserver" section to the top of the "Running web app/landing page/mobile app" sections, made some rewording and clarifying changes. More details in inline comments.

Test Plan: N/A

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, atul

Differential Revision: https://phab.comm.dev/D4414