docs: add AmigaOS release readme

[boingball]

Motivation

• Provide a dedicated AmigaOS README describing the curl/libcurl 8.18.0-DEV binary archive, runtime requirements and CPU-targeted libraries.
• Make Amiga-specific build/configuration notes and release highlights discoverable from the existing AmigaOS install instructions.
• Ensure the AmigaOS notes are packaged with the repository documentation distribution.

Description

• Add docs/AMIGAOS.md containing the archive banner, supported protocols/features, runtime requirements (AmiSSL version, AmigaOS versions, STACK guidance), tested TCP environments, CPU-specific binaries and libcurl naming, a sample autotools configuration and release notes.
• Update docs/INSTALL.md to reference docs/AMIGAOS.md from the AmigaOS section.
• Add AMIGAOS.md to EXTRA_DIST in docs/Makefile.am so the file is included with the documentation distribution.

Testing

• Ran git diff --check which passed with no whitespace errors.
• Ran ./scripts/mdlinkcheck --dry-run docs/AMIGAOS.md which completed with no link errors detected.
• Ran perl scripts/checksrc.pl docs/AMIGAOS.md docs/Makefile.am which reported style checks (4 errors and 20 warnings) and needs follow-up to fully satisfy repository checksrc rules.

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Codex Task