profile: surface disclose.io video content (YouTube badge + Watch grid)

[yesnet0]

Surface disclose.io's video content on the org profile

Adds the project's video presence to the org profile README, matching the existing badge/HTML idiom.

Changes

• YouTube badge added to the header badge row → @discloseio.
• New 🎥 Watch section — a 2×2 thumbnail grid of the four talks disclose.io already features on its own homepage (so the editorial picks are pre-endorsed, not new claims):
  • An intro to disclose.io & hacker safety — @disclose_io, #HackerCon 2021
  • Hacking the Law: Are Bug Bounties a True Safe Harbor? — Amit Elazari, BSidesSF 2018
  • Cybertalks 2017 — Leonard Bailey · Casey Ellis · Marten Mickos, CyberScoop
  • Hacking Policy and Policy Hacking (Keynote) — Amit Elazari, BSidesSF 2023
• YouTube link added to the "Get involved" footer row.

Note on Vimeo: there is no disclose.io Vimeo channel — vimeo.com/{discloseio,disclose,disclose-io} all 404. The only match found was a personal account, which I deliberately did not add to the org. If a real disclose.io Vimeo exists, drop the URL and I'll add it.

Note on the channel: @discloseio is real but dormant (≈3 uploads, 2020–21). The Watch grid leans on the curated talks rather than the channel's thin upload list, so the section is strong regardless.

Thumbnails use img.youtube.com/vi/<id>/mqdefault.jpg; all four verified 200. No changes outside profile/README.md.