[Improvement] Add instruction for custom documentation, regenerate embeddings

[cbtham]

What kind of change does this PR introduce?

Documentation update

What is the current behavior?

No instruction on how to supply custom mdx documentation.

What is the new behavior?

Documentation detailing how to add custom mdx and re-process the embeddings

[cbtham]

Thanks @gregnr for reviewing! I have taken your suggestions and made a few corrections.

On a separate note, the localhost QnA page doesn't seem to render code blocks correctly (eg. ``` sql SELECT * FROM EMP JOIN DEPT ``` ).

Is this by design in this template?

[gregnr]

Hey @cbtham, thanks for the update. Would you mind pointing me to which QnA page you are referring to?

Code blocks should have this format:

```sql
SELECT * FROM EMP;