Improve docs: user-documentation/en/collect/jabref-browser-extension.md

I have installed jabref browser extension on a linux system twice in last week, once for myself and once for a friend.

Unfortunately the documentation for the browser installation feels not very user friendly and confusing. Even at the second installation (and native host configuration) for jabref we had to take a deep look into the docs at https://docs.jabref.org/collect/jabref-browser-extension#installation-and-configuration

I feel the docs for browser extension could really benefit from more structure. For example a sub-page for each operating system and then proper headlines for different browser configurations. Or a sub-page for each browser and then a proper headline for each operating system.

But right now the "Manual Installation for Chrome on Linux" is a mess to find far down in a listing:

I also feel that the twice-repeated call to download is not really a clear call to action and more confusing than it should be:

Lastly it should be more clear from the start that the installation takes three steps:

1. install browser extension
2. install native message handler
3. ensure that your jabrefHost.py or .bat is in the proper location

I see from other docs such as https://docs.jabref.org/collect/import that further sub-categories are possible.

So I would like to ask you:

• Would you be willing to have more sub-pages for this doc page?
• If yes, should it be sub-categories by operating system or by browser? (I think it should be by browser)

If we get a consensus on these questions I'm happy to contribute a PR.

Thank you!

Review user-documentation/en/collect/jabref-browser-extension.md and its installation-and-configuration section first. A completed documentation change would make the three installation steps and the browser or operating-system instructions easier to find, but the issue still asks for agreement on the page organization before implementation.