chore: Write README for all packages except tokens

[VincentSmedinga]

Describe the pull request

What

This is a continuation of #1655 to improve the general documentation of all our packages.

It also removes mentions of Salesforce, as we no longer aim to replicate our components for it. I’ve removed more ‘future plans’ and ‘roadmaps’ as they easily get outdated.

Why

To help people understand what we offer and how they can use it.

How

Adds or updates the README and some fields in package.json for the assets, css, react, and react-icons packages.

The sections headings are consistently as follows, with all of them optional:

1. About
2. Motivation
3. Installation
4. Usage
5. Updating
6. Support

Checklist

Before submitting your pull request, please ensure you have done the following. Check each checkmark if you have done so or if it wasn't necessary:

• Add or update unit tests
• Add or update documentation
• Add or update stories
• Add or update exports in index.* files
• Start the PR title with a Conventional Commit prefix, as explained here.

Additional notes

• Provide links to any related issues or discussions.
• Add a link to the specific story in the feature branch deploy.
• Mention any areas where additional review or feedback is needed.