Update HexDoc URL

[fhunleth]

Migrates HexDocs URLs to the 2026 format using hex_url_migrator.

⚠️ URL verification warnings from hex_url_migrator --verify

❌ Verification failed for https://circuits-uart.hexdocs.pm/Circuits.UART.html): Status 404
❌ Verification failed for https://circuits-uart.hexdocs.pm/): Status 404
❌ Verification failed for https://elixir.hexdocs.pm/Port.html): Status 404
⚠️ Verification finished: 1 success, 3 failure(s).

Reported by the migrator's verifier. Note that 301s are typically redirects (the target exists) and a trailing ) is a markdown-link parsing artifact. Please review before merging.