Docs: Add new user guide to documentation

This merge requests adds a new user guide to the documentation. It adds two pages to the Getting Started chapter, add a new podcast and add a new episode. The user guide contains 3 sections: managing the instance, managing podcasts, and a website overview. All the content is included except for a page on Monetization.

I successfully added two pages to the left hand navigation under Getting Started and there is a landing page for the User Guide. However, the two pages I added to the sidebar under the User Guide in astro.config.mjs are not showing in the sidebar. I could use some help fixing that and once fixed I can add the rest of the pages to the sidebar.

Marking this as a draft until the menu is fixed.