[docs] Rearrange "Running keyserver" and "Running web app/landing page"

Summary:
Relevant Linear issue with full context here. Rearranges the "Running keyserver" and "Running web app/landing page" sections and also adds a short sentence about making sure the keyserver is running in the web app, landing page, and mobile app sections. More details in inline comments.

Aside, but viewing docs diffs with the Phabricator diff tool (and really most diff tools) can be kind of a headache since it's a text file, not code. I know we can't control the diff tool on Phabricator, but here are some cool options for diff tools that reduce the amount of green/red you have to parse together to understand what was changed and what remained the same.

Test Plan: N/A

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, atul

Differential Revision: https://phab.comm.dev/D4399