[mdatagen] Generate documentation for components with resource attributes only

[ChrsMark]

Description

This PR adds support for generating documentation.md files for components which only define resource attributes.
This was reported at #10705 and also applies at open-telemetry/opentelemetry-collector-contrib#34210.

Link to tracking issue

Fixes #10705

Testing

Tuned tests and added a sample processor component which only defines resource attributes.

Documentation

[codecov]

Codecov Report

Attention: Patch coverage is 92.04545% with 7 lines in your changes missing coverage. Please review.

Project coverage is 91.47%. Comparing base (52e11ec) to head (b3e22ec).

Files with missing lines	Patch %	Lines
...pleprocessor/internal/metadata/generated_config.go	88.23%	2 Missing and 2 partials ⚠️
...eprocessor/internal/metadata/generated_resource.go	91.42%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11242   +/-   ##
=======================================
  Coverage   91.47%   91.47%           
=======================================
  Files         424      427    +3     
  Lines       20206    20293   +87     
=======================================
+ Hits        18484    18564   +80     
- Misses       1339     1344    +5     
- Partials      383      385    +2     

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