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 83.29%. Comparing base (c089ea8) to head (2dce50d).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1415      +/-   ##
============================================
- Coverage     84.47%   83.29%   -1.18%     
- Complexity      244      341      +97     
============================================
  Files            28       37       +9     
  Lines          1288     1634     +346     
  Branches        157      239      +82     
============================================
+ Hits           1088     1361     +273     
- Misses          140      172      +32     
- Partials         60      101      +41     

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