Merge cell count and fusion nomenclature-related changes to `main`

[jaclyn-taroni]

Purpose

• Removing sample_cell_count_estimate from description of metdata
• Adding a description of demux_sample_cell_estimate to multiplexed downloadable files section
• Adding a CHANGELOG entry that covers the above + the standardization of fusion nomenclature

Issue addressed

357

Checklist for publishing to main

• [x] This branch contains all commits/content ready to be released.
• [x] The most recent CHANGELOG entry includes a date. If this release required a CHANGELOG entry, it should be today's date.