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 80.11%. Comparing base (d25996e) to head (b56443e).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #757   +/-   ##
=========================================
  Coverage     80.11%   80.11%           
  Complexity      109      109           
=========================================
  Files            41       41           
  Lines          3933     3933           
  Branches        666      666           
=========================================
  Hits           3151     3151           
  Misses          526      526           
  Partials        256      256           

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