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 87.71%. Comparing base (2ae4d59) to head (7ad3d23).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #300   +/-   ##
=========================================
  Coverage     87.71%   87.71%           
  Complexity      373      373           
=========================================
  Files            30       30           
  Lines          1188     1188           
  Branches        149      149           
=========================================
  Hits           1042     1042           
  Misses           92       92           
  Partials         54       54           

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