Updated installation instructions

The installation instructions at https://moment.github.io/luxon/#/install can be somewhat misleading and/or outdated:

• A user in a different issue thought you always needed to download and update files to use them for ESM imports. This is probably since we only provide files and don't provide commands.
• Both React Native versions currently mentioned are unsupported.

My suggestions:

• Most web developers today build some kind of app (create-react-app, vite etc.). Maybe provide instructions for them first, like most js librarys do?
• Perhaps the three most popular node package managers should be mentioned - yarn, npm and pnpm. Also, the --save flag is no longer needed for npm - packages are installed as a non-dev dependency per default.
• Continue to provide downloads for users that really need them, but further down on the page.
• Provide a warning that we are seeing multiple issues with React Native not providing full Intl support. Inform the user to expect some issues and that polyfills might be needed.