Update broken links in repo README

Problem/Motivation

The README points to 404 pages for several links to documentation: https://git.drupalcode.org/project/civictheme

Steps to reproduce

1. Go to: https://git.drupalcode.org/project/civictheme
2. Scroll down to the heading 'Installation'
3. Click on the link https://docs.civictheme.io/development/drupal-theme
4. Scroll further and click on the Sub-theme link
5. Scroll further and click on the Version update link
6. Scroll further and click on the Colour selector link

Observed

The links are broken. This looks like it is due to the URL being updated.

Proposed resolution

Update the links or remove the links as follows:

• Installation link: Update link: https://docs.civictheme.io/installation/drupal-theme
• Sub-theme: Update link: https://github.com/civictheme/docs/blob/main/development/drupal-theme/su...
• Version update: Delete this section
• Colour selector: Delete this section