post_installation.md: Adding a .ipynb to test the pipeline's ingested documents

[burrsutter]

As of this moment, the post_installation.md has the reader review the number of rows in the pgvector database via
SELECT COUNT(*) FROM vector_store_rag_vector_db;
which I do really like, however, this step assumes the reader is a database oriented person while our typical RHOAI end-user will be more of a Notebook oriented person. Therefore, I believe we need another .ipynb that is has the code to test against the ingested data. Allowing the reader to replace terms/prompts with content they believe to be in their ingested PDFs.

Also, we should have a downloadable PDF that allows our docs to refer to something known and giving the reader the opportunity to have a PDF that is known to work well.

[acmenezes]

Hello @burrsutter I pushed #71 that may kind of do what you expect here. And #70 uploads a known pdf. We can improve this certainly. Let us know any improvements you would like to see here.