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 88.19%. Comparing base (42001e5) to head (010e9b5).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1061   +/-   ##
=========================================
  Coverage     88.19%   88.19%           
  Complexity       37       37           
=========================================
  Files            13       13           
  Lines           788      788           
  Branches        221      221           
=========================================
  Hits            695      695           
  Misses           90       90           
  Partials          3        3           

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