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 90.35%. Comparing base (cb84357) to head (2a269c4).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #512   +/-   ##
=========================================
  Coverage     90.35%   90.35%           
  Complexity       92       92           
=========================================
  Files            11       11           
  Lines           342      342           
  Branches         41       41           
=========================================
  Hits            309      309           
  Misses           29       29           
  Partials          4        4           

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