Update the documentation

Problem/Motivation

It would be great to update the documentation.

The README could be updated, as well as the documentation link on the project page.

Steps to reproduce

Proposed resolution

• ☑ Create Add icon to flag link with CSS example doc page
• ☑ Update link to Drupal 10 doc page on project page https://www.drupal.org/project/flag
  From: http://drupal.org/handbook/modules/flag
  To: Documentation
• ☑ Add link to CSS example in README (See MR below)
• ☑Create "Code Examples" doc pages and add it to the menu
• ☐ Feel free to add more tasks :)

Remaining tasks

User interface changes

API changes

Data model changes