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 89.54%. Comparing base (98689e2) to head (72cf8c3).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #131      +/-   ##
============================================
+ Coverage     88.34%   89.54%   +1.19%     
- Complexity      226      315      +89     
============================================
  Files            22       38      +16     
  Lines           489      679     +190     
  Branches        102      134      +32     
============================================
+ Hits            432      608     +176     
- Misses           36       41       +5     
- Partials         21       30       +9     

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