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 85.35%. Comparing base (2e1a3ca) to head (dbc7dd0).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1153   +/-   ##
=========================================
  Coverage     85.35%   85.35%           
  Complexity        3        3           
=========================================
  Files             4        4           
  Lines           157      157           
  Branches         44       44           
=========================================
  Hits            134      134           
  Misses           22       22           
  Partials          1        1           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests