[audit/high] Mark staleness with last-verified timestamps

[Displacedtex]

Priority: High

Problem

Many README entries date from 2020–2021 era content (rows 24–32 are data-flair.training "2021" posts). Today's visitors have no signal about whether a link is still worth clicking.

Recommendation

Choose one:

• Add a Last verified column to each table / (verified YYYY-MM) badge per entry.
• Add a per-category footer: Last audited: YYYY-MM.
• Run the link-check workflow ([audit/critical] Add automated link-rot protection (CI) #3) weekly and stamp README.md via an automated commit.

Acceptance criteria

• Readers can tell at a glance which entries have been verified recently.
• Entries that 404 are pruned or clearly flagged.

Part of the repo audit backlog. Blocks effective follow-through on #3.