Add insta-dl to Instagram section

[subzeroid]

Summary

• Add insta-dl to the Instagram tools section, next to instaloader and insto.
• Link to the public GitHub repo and summarize its Instagram downloader coverage, metadata output, timestamp preservation, and pluggable backends.

Checks

• git diff --check
• curl -I -sS https://github.com/subzeroid/insta-dl -> HTTP 200
• npm test -> fails on pre-existing README markdownlint issues across the repository before this one-line addition (for example, README.md:1 MD033/no-inline-html and many existing MD013 line-length violations).