fix: Add guided install scope and intended use docs

[Nandu-pns]

Jira - https://new-relic.atlassian.net/browse/NR-520228

Update documentation to clearly communicate that CLI-based guided install is designed for initial installations on new hosts, and that running it on systems with existing agents will reset configurations to defaults.

Changes:

• Added "Guided Install: Scope and Usage" section to README
• Created comprehensive INSTALL_GUIDE.md with detailed guidance
• Update newrelic_install.md with "Important: Intended Use" section
• Added guided install reference to GETTING_STARTED.md
• Fix broken community forum links (forum.newrelic.com unavailable)
• Clarify upgrade procedures and when to use manual vs guided install

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.33%. Comparing base (5d41739) to head (34e0dff).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1792   +/-   ##
=======================================
  Coverage   35.33%   35.33%           
=======================================
  Files         156      156           
  Lines       10677    10677           
=======================================
  Hits         3773     3773           
  Misses       6763     6763           
  Partials      141      141           

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