Add supported URL reference doc

[rutul2006]

What changed

Fixes (#25)

How to test

Visual Changes

Quality Checklist

Testing Locally

• [] The app still loads and works end-to-end (paste a URL, generate, download).
• [] Your change works on mobile (Chrome DevTools → responsive mode).
• [] Your change works in Chrome, Firefox, and Safari (if relevant).
• [] No new console errors or warnings are generated.
• [] If you changed PDF output, the generated PDF looks correct.
• [] If you changed the UI, it still matches the overall dark aesthetic.

Code Quality

• [] My branch is completely up to date with main.
• [] No commented-out debug code has been left in.
• [] CSS variables are used for all colors (no hardcoded hex values that break theming).
• [] All changes remain within index.html (unless strong architectural reasons were approved).

GSSoC '26 Compliance

• [] This issue was officially assigned to me before opening this PR.
• [] I have labeled this PR with gssoc26 in the description.
• [] This communication and my code adjustments are completely my own work.

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