The section about activity watchers for browsers should point directly to chrome store instead of github

[tmotyl]

currently there is :

Watches properties of the active tab like title, URL, and incognito state.

[aw-watcher-web](https://github.com/ActivityWatch/aw-watcher-web) - The official browser extension, supports Chrome and Firefox.

instead of linking to github, docs should link to app stores for given browsers:
e.g. https://chrome.google.com/webstore/detail/activitywatch-web-watcher/nglaklhklhcoonedhgnpgddginnjdadi/related