docs: make Customization a subtopic under Configuration (Fixes #1663)

[vivekkumarrathour]

Description

This PR reorganizes the documentation so that Customization appears as a subsection under Configuration, as suggested in issue #1663. The previous navigation structure listed both topics at the same level, which caused confusion because customization options are part of configuration.

What I changed:

• Updated mkdocs.yml to nest Customization under Configuration
• Adjusted the heading in docs/customization.md to match its new position in the hierarchy

Fixes #1663

Checklist

• [x] I have read the contributing guidelines

Code Changes

N/A — this PR only updates documentation structure.

Documentation Changes

• [x] Updated navigation structure in mkdocs.yml
• [x] Adjusted heading levels in customization.md
• [x] Verified the documentation builds correctly

Expected Behavior

The documentation sidebar should now display:

Configuration
↳ Overview
↳ Customization

This better reflects the intended hierarchy.

Steps to Test This Pull Request

1. Run mkdocs serve
2. Check the left sidebar to confirm that:
   • "Customization" is listed under "Configuration"
   • All pages load correctly without broken links

Additional Context

No further changes required.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 98.68%. Comparing base (120d514) to head (fce8dfe). :warning: Report is 885 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1674      +/-   ##
==========================================
+ Coverage   97.33%   98.68%   +1.34%     
==========================================
  Files          42       60      +18     
  Lines        2104     2656     +552     
==========================================
+ Hits         2048     2621     +573     
+ Misses         56       35      -21     

Flag	Coverage Δ
unittests	98.68% <ø> (+1.34%)	:arrow_up:

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[bearomorphism]

Actually I already created #1657 which splits configuration.md into pages.

[bearomorphism]

Closing since the issue is already resolved.