Update the docs for writing schemas

[HardikGoyal2003]

Many of the details on this page are outdated, making it difficult for new users to understand, leading to confusion. Therefore, the schemas page needs to be updated.

Examples:

• The file below does not exist.
  

• This code reference also does not exist.
  

Additional issues are likely present, so a thorough review and update are necessary.

[AadityaBansal01]

Hi! I'd like to work on this issue. Could you assign it to me, please?

[seanlip]

@AadityaBansal01 Per the guidance at https://github.com/oppia/oppia/wiki/Contributing-code-to-Oppia#choosing-a-good-first-issue, please provide an explanation of what your PR will do (with names of files you're changing, what you plan to change in each file, etc.). If it looks good, we can assign you to this issue.

Please also follow the other instructions on that wiki page if you have not yet done so. Thanks!