fix: create index from single document

[jiashenC]

Use a runtime row_number to build the index by incorporating design discussions from #912 and #868.

[jiashenC]

@aryan-rajoria Please pull this branch to test if issue of creating index from single PDF is fixed.

[xzdandy]

Not relevant to this PR. Are we still having incorrect links in documentation?

sphinx.errors.SphinxWarning: /home/circleci/project/docs/source/overview/concepts.rst:57:broken link: https://github.com/georgia-tech-db/evadb/blob/master/evadb/udfs/mnist_image_classifier.py (404 Client Error: Not Found for url: https://github.com/georgia-tech-db/evadb/blob/master/evadb/udfs/mnist_image_classifier.py)

[jiashenC]

Not relevant to this PR. Are we still having incorrect links in documentation?

sphinx.errors.SphinxWarning: /home/circleci/project/docs/source/overview/concepts.rst:57:broken link: https://github.com/georgia-tech-db/evadb/blob/master/evadb/udfs/mnist_image_classifier.py (404 Client Error: Not Found for url: https://github.com/georgia-tech-db/evadb/blob/master/evadb/udfs/mnist_image_classifier.py)

Probably because I haven't rebased with staging. Let me sync with it.