Support custom md files for package documentation

[azinneera]

Purpose

Support custom md files for package documentation

Fixes #42981

Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[codecov]

Codecov Report

Attention: Patch coverage is 94.91525% with 6 lines in your changes missing coverage. Please review.

Project coverage is 77.37%. Comparing base (8e7d9f6) to head (00f0129).
Report is 107 commits behind head on master.

Files	Patch %	Lines
...o/ballerina/projects/internal/ManifestBuilder.java	80.00%	1 Missing and 4 partials ⚠️
...rc/main/java/io/ballerina/projects/BalaWriter.java	98.61%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #43201   +/-   ##
=========================================
  Coverage     77.36%   77.37%           
- Complexity    58372    58399   +27     
=========================================
  Files          3453     3453           
  Lines        219275   219350   +75     
  Branches      28801    28823   +22     
=========================================
+ Hits         169650   169720   +70     
+ Misses        40267    40265    -2     
- Partials       9358     9365    +7     

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