Improve README.rst readability with TOC and clearer instructions

[Brandie-M]

The current README.rst file for the Astropy project can be improved for better readability and usability.

The following changes are proposed:

1. Add Table of Contents: A Table of Contents (TOC) will be added to provide easy navigation through the README document. This will help users quickly find the sections they need.
2. Update Installation Instructions: Include a brief example command for installing Astropy using pip. This will provide immediate, clear instructions for users who may already be familiar with the package and simply want to install. Links to the full documentation remain.
3. Clean Up Local Clone Instruction Heading: Improve the heading for the instructions for updating local clones from master to main for better readability.

Impact:
These changes will enhance the visual appeal and usability of the README.rst file, making it more user-friendly for contributors and users.

Screenshot of proposed changes:

[github-actions]

Welcome to Astropy 👋 and thank you for your first issue!

A project member will respond to you as soon as possible; in the meantime, please double-check the guidelines for submitting issues and make sure you've provided the requested details.

GitHub issues in the Astropy repository are used to track bug reports and feature requests; If your issue poses a question about how to use Astropy, please instead raise your question in the Astropy Discourse user forum and close this issue.

If you feel that this issue has not been responded to in a timely manner, please send a message directly to the development mailing list. If the issue is urgent or sensitive in nature (e.g., a security vulnerability) please send an e-mail directly to the private e-mail feedback@astropy.org.