docs: add install section on README

[SethFalco]

Clarify how to install the tool locally before the usage instructions. I also reworded parts of the README in general.

Chores

I also did some chores while I was here:

• Subjectively reorganizes the package.json.
• Upgrade the translations job to use Node.js v20.
• Omit the version number from the job name, as it's unnecessary.
• When parsing a page, if the page's layout is unexpected, call it malformed instead of invalid so it's clearer the content of the page is the problem, as opposed to the page not existing for example.