Compress remaining large used images at JPEG quality 80, remove unused ones

The prior quality-90 compression pass left several used PNGs
unconverted because JPEG-90 wasn't smaller for them (mostly diagram/
illustration-style images with flat color regions). Re-ran at
quality=80 for whatever is still PNG: 17 more converted (2.57MB ->
1.40MB on that set), 3 stayed PNG (JPEG-80 still wasn't smaller), 1
skipped for real alpha transparency (images/bentoml.png). Markdown
references updated as before.

Also removes 179 genuinely unused tracked images (21.9MB) found by
the same basename-reference audit used throughout this session --
these were left alone in the prior pass as out of scope, but the
repository is currently blocked from being ingested by the website's
100MB total-tracked-bytes limit (106.7MB before this commit) with the
2026 cohort starting in 4 days, so removing confirmed-dead weight is
the direct fix. Verified via repo-wide grep that none of the 179 is
referenced anywhere before removal.

Total tracked size: 106.7MB -> 83.6MB, under the 100MB pipeline limit
with headroom.

Author: alexeygrigorev