Remove legacy BETY installation content from dedicated install page (#3744)

[ANAMASGARD]

Fixes #3744

Description

Replaces the deprecated BETY installation page with a short Docker-first router page and removes all legacy manual installation content from 05_install_BETY.Rmd.

Removed from the dedicated BETY install page:

• Deprecation banner followed by full legacy instructions
• Manual PostgreSQL / load.bety.sh database setup
• PHP and Ruby/Rails web application installation steps (gem, Bundler, config files, etc.)

Added instead:

• Brief explanation of what BETYdb is
• Single link to the PEcAn Docker installation guide
• Link to the BETYdb Technical Guide for standalone or bare-metal installs
• Short note explaining why legacy content was removed

Also removes the stale THIS PAGE IS DEPRECATED label from the OS install navigation in 00_install_OS.Rmd. The {#install-bety} anchor is preserved so existing links (e.g. from README.md) continue to work.

Motivation and Context

The BETY install page previously warned that it was deprecated but still contained extensive PHP/Ruby/manual setup instructions. That created conflicting installation paths and maintenance burden, especially given outdated Ruby/Rails versions.

Per maintainer consensus on #3744:

• PEcAn’s dedicated BETY install page should be Docker-first
• Bare-metal installation instructions should live in the BETY Technical Guide, not be duplicated here

This PR addresses the dedicated BETY page only. Legacy BETY material on Ubuntu/CentOS/macOS install pages and the broken [Installing BETY] reference in 05-compile-pecan.Rmd are intentionally left for follow-up work.

Review Time Estimate

• When possible

Types of changes

• Documentation only (non-breaking documentation cleanup)

Checklist:

• My change requires a change to the documentation.
• I agree that PEcAn Project may distribute my contribution under any or all of
  • the same license as the existing code,
  • and/or the BSD 3-clause license.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• All new and existing tests passed (N/A for docs; renderbook CI expected to validate)

Test plan

• Legacy patterns absent from 05_install_BETY.Rmd (source grep)
• #install-bety, #install-docker, and BETY Technical Guide URL present in source
• THIS PAGE IS DEPRECATED removed from nav
• bookdown::render_book() succeeds locally

[ANAMASGARD]

All documentation-relevant checks pass (renderbook, CI, pkgdown).

The 6 failing Docker GHA jobs appear unrelated to this docs-only change —
they fail on GHCR push (denied: installation not allowed to Write organization package)
for fork PR tags like ghcr.io/pecanproject/depends:pr-4055. The image builds complete;
only push is denied. Happy to leave Docker CI hardening to a separate issue if maintainers agree.