Prevent editors from publishing incomplete contents

[Riley1101]

Issue:
Right now, our editors can post things without finishing all the necessary details.
Then the website will break lol.

Let's fix that by making certain parts required before they can publish.

Changes:

1. Pick out the important details that need to be filled in ‌ and set them as required in sanity schema.

Context:
This will help us have better, complete content every time something is posted. And of course breaking the website lol

Note to Maintainers:
Think about doing this to make sure the content posted is always complete and good quality.

Resources
Useful resources

• https://www.sanity.io/docs/validation