chores: revise readme and package metadata

[SethFalco]

I've just released v1.0.0 of browserslist-plausible, however, I'd like to make a few small revisions to the README/metadata before sharing it further.

As it's just many minor edits, I've done it all in a single commit.

README

• Include a npm version badge/shield in the README. (Since each badge is +10% performance.)
• Drop the Overview section, this felt cluttered.
• When using browserslist-plausible as a noun, I've removed the inline code syntax. (browserslist-plausible → browserslist-plausible)
• When recommending how to install it globally, I add --help to the end to make it a valid command, and guide the reader toward the help information.
• Give a clearer heading to Get an API Key. I wanted it to refer to explicitly what the reader would be getting an API key for to avoid misunderstandings when skimming the document.
• When using browerslist-plausible, use npx rather than an assumed global installation.

Metadata

It's redundant to link to the GitHub repository twice. I originally removed Homepage, but then figured we could just point it to browserl.ist.