Add BM25Encoder.update() which updates encoder values with new documents

[adrwz]

Problem

If we want to use this BM25Encoder in production & host it somewhere (or save it to a .pkl), there's no way to update it with new documents.

Solution

Wrote update() to update the BM25 encoder with documents.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

Run update()