Add user_fx installation instructions and a usermod config example

[gustebeast]

When I was getting started I had trouble with WLED information referencing old methods for installing usermods. Since user_fx is a form of example/getting started effect it seems worth explaining the process again to be clear.

I also just had difficulty figuring out how to read usermod config values from my effect function, and having an example for that would have helped a lot.

Summary by CodeRabbit

• Documentation

  • Added an Installation guide with activation instructions and examples; expanded navigation with sections on metadata, effects, combining effects, and compilation.

• Chores

  • Added developer scaffolding and commented reference blocks showing where to add persistent configuration read/write guidance (documentation-only; no runtime or behavioral changes).

[coderabbitai]

Walkthrough

Adds an Installation section and navigation entries to usermods/user_fx/README.md, and inserts commented configuration scaffolding and examples in usermods/user_fx/user_fx.cpp. No executable code, public API signatures, or runtime behavior were changed. (50 words)

Changes

Cohort / File(s)	Summary
Documentation usermods/user_fx/README.md	Added "Installation" TOC entry and new Installation section with platformio_override.ini examples and alternative wording for including the usermod.
Commented config scaffolding usermods/user_fx/user_fx.cpp	Inserted commented guidance showing where to add class-level config fields and example addToConfig / readFromConfig implementations; comments only, no active code or API changes.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• Added new effect usermod - Diffusion Fire #4667: Modifies the same usermods/user_fx area (adds a new effect and related files), likely related by targeting the same usermod.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and concisely describes the main changes: adding installation instructions to the README and including a usermod configuration example in the source code.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@usermods/user_fx/README.md`:
- Around line 18-22: Update the Installation section to show the setting as an
.ini code block and demonstrate both the simple add and the safe append case so
users don't overwrite existing usermods; explicitly reference the
platformio_override.ini key name custom_usermods and the usermod name user_fx,
show the single-entry example (custom_usermods = user_fx) inside an ini block,
and add a second ini block showing how to append to an existing list (e.g.,
custom_usermods = user_v2, user_fx).