Rename column_index to cell_index in KZG spec

[jtraglia]

Primarily, this PR renames some of the index params in the polynomial commitment spec. We do this because KZG libraries should have no concept of "row" or "column". The core DAS spec will still use RowIndex and ColumnIndex types.

Notably:

• column_indices -> cell_indices
• row_indices -> commitment_indices
• row_commitments -> commitments

It also does some other small things like:

• Add a new CommitmentIndex type.
• Improve compute_verify_cell_kzg_proof_batch_challenge function.
• Remove punctuation from MAX_CELLS_IN_EXTENDED_MATRIX description.
• Rename constants section which had no constants.

[asn-d6]

Some minor comments, and then LGTM.

[b-wagn]

Thanks for the PR, looks good.
I left some minor comments.