Support custom md files for package documentation

[azinneera]

Purpose

Introduce README.md as the default documentation file for packages.

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 83.87097% with 70 lines in your changes missing coverage. Please review.

Project coverage is 77.47%. Comparing base (e8b5bc5) to head (fb27ea2).
Report is 257 commits behind head on master.

Files with missing lines	Patch %	Lines
...o/ballerina/projects/internal/ManifestBuilder.java	82.67%	11 Missing and 11 partials ⚠️
...rc/main/java/io/ballerina/cli/cmd/CommandUtil.java	61.53%	15 Missing and 5 partials ⚠️
...ballerina/projects/internal/model/PackageJson.java	33.33%	6 Missing ⚠️
...java/io/ballerina/projects/internal/BalaFiles.java	86.48%	3 Missing and 2 partials ⚠️
...rc/main/java/io/ballerina/cli/cmd/PushCommand.java	84.61%	2 Missing ⚠️
...rc/main/java/io/ballerina/projects/BalaWriter.java	94.59%	0 Missing and 2 partials ⚠️
...in/java/io/ballerina/projects/PackageReadmeMd.java	90.47%	2 Missing ⚠️
...ava/io/ballerina/tools/diagnostics/Diagnostic.java	60.00%	1 Missing and 1 partial ⚠️
...allerinalang/docgen/generator/model/ModuleDoc.java	33.33%	1 Missing and 1 partial ⚠️
.../main/java/io/ballerina/projects/ModuleConfig.java	66.66%	1 Missing ⚠️
... and 6 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #43497      +/-   ##
============================================
- Coverage     77.55%   77.47%   -0.09%     
+ Complexity    58752    58512     -240     
============================================
  Files          3448     3434      -14     
  Lines        219728   218711    -1017     
  Branches      28922    28918       -4     
============================================
- Hits         170411   169443     -968     
+ Misses        39921    39874      -47     
+ Partials       9396     9394       -2     

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