Make README.md as the default package documentation

[NipunaRanasinghe]

This PR includes the following changes:

• Rename Module.md to README.md in the default module and submodules
• Remove Package.md from the default module

Related to ballerina-platform/ballerina-library#7404

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.07%. Comparing base (71d3277) to head (201bbd1).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1287   +/-   ##
=========================================
  Coverage     85.07%   85.07%           
  Complexity      128      128           
=========================================
  Files            21       21           
  Lines           583      583           
  Branches         49       49           
=========================================
  Hits            496      496           
  Misses           66       66           
  Partials         21       21           

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