Add Shelf support

[amydosomething]

Fiverr is a global freelance marketplace where users offer and purchase digital services.

Substack is a newsletter and blogging platform that allows writers to publish content and build paid subscriptions.

Shelf is a social bookmarking platform where users curate and share collections of their favorite books, artists, genres, etc.

[github-actions]

Automatic validation of changes

Target	F+ Check	F- Check
Substack	❌   Fail	❌   Fail
Shelf	✔️   Pass	✔️   Pass
Fiverr	❌   Fail	✔️   Pass

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Failures were detected on at least one updated target. Commits containing accuracy failures will often not be merged (unless a rationale is provided, such as false negatives due to regional differences).

[github-actions]

Automatic validation of changes

Target	F+ Check	F- Check
Shelf	✔️   Pass	✔️   Pass

[amydosomething]

Update: I've removed Fiverr and Substack from this PR.

1. For Substack, running into Cloudflare/WAF protection that blocks GitHub Actions IPs. I've verified locally from my machine and it works perfectly, but the CI environment gets blocked with WAF errors.

2. Checked removed-sites.md for Fiverr and realised that it was already removed from Sherlock (2025-02-02) due to CSRF protections.

Shelf.im passes both validation tests.

[ppfeister]

Appreciate the contribution!

All LGTM. Let's get this merged.