bf-10qd4: Mark unpublished install channels as coming-soon

Summary

Fixed README.md to clearly indicate all unpublished install channels, addressing the issue where install commands pointed to nonexistent artifacts.

crates.io: No pdftract or pdftract-core exists (confirmed via web search)
PyPI: No pdftract package exists (confirmed via web search)
Docker Hub: No ronaldraygun/pdftract image exists (confirmed via web search)
Homebrew: No pdftract formula exists (confirmed via web search)
Git tags: No version tags exist (confirmed via git tag)

Badges (lines 3-4): Changed crates.io and PyPI badges from version badges to "coming-soon" badges
Installation section (lines 39-72):
- Added prominent warning: "🚧 Public releases coming soon"
- Moved development installation (build from source) to the top
- Added "Planned release channels (coming soon)" subsection
- Marked each install command with status: "Status: Not yet published to [platform] — tracked in bf-10qd4"
Distribution table (lines 233-239): Added "🚧 coming soon" to all unpublished SDKs

Commit: 7a1b0d43 - docs(bf-10qd4): mark unpublished install channels as coming-soon
Pushed to origin (git.ardenone.com) successfully

PASS: All acceptance criteria met:

Details:

All install commands (cargo, pip, docker, brew) are now marked with "Status: Not yet published..." notes
Users are directed to build from source instead of attempting to install from nonexistent packages
Badges now show "coming-soon" status instead of non-existent version numbers
Distribution table clearly indicates which SDKs are not yet published

Can verify by:

Checking that no install command in README.md points to an artifact without a "coming-soon" or status disclaimer
Confirming all badges point to valid URLs (docs.rs badge retained, others redirected to plan)
Building from source using the new "Development installation" instructions