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 90.22%. Comparing base (9b834bd) to head (f92ee50).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #548   +/-   ##
=========================================
  Coverage     90.22%   90.22%           
  Complexity      248      248           
=========================================
  Files             9        9           
  Lines           870      870           
  Branches        239      239           
=========================================
  Hits            785      785           
  Misses           48       48           
  Partials         37       37           

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