(#1065) Add Highlight information to readme

[st3phhays]

Description Of Changes

This adds information on how to add a new Highlight to both the left sidebar and the home page of docs.chocolatey.org.

Motivation and Context

This process was not documented with the switch to Astro.

Testing

The best way to test this would be to go through the steps added to the readme, and try adding a "hypothetical" Home Highlight and Sidebar Highlight, and note if any of the instructions are not clear or confusing.

• I have previewed these changes using the Docker Container or another method before submitting this pull request.

Change Types Made

• Minor documentation fix (typos etc.).
• Major documentation change (refactoring, reformatting or adding documentation to existing page).
• New documentation page added.
• The change I have made should have a video added, and I have raised an issue for this.
  • Issue #

Change Checklist

• Requires a change to menu structure (top or left-hand side)/
• Menu structure has been updated

Related Issue

• Add information on how to create a new highlight to the readme #1065

Fixes #1065