Fix Javadoc link in Web API basics guide

[Sandhyasteffy]

Fixes #1591

The Javadoc link in the Web API basics guide was pointing to the old Sonatype repository URL, which no longer works.

This PR updates the link to the corresponding Javadoc.io page.

Category

• bolt (Bolt for Java)
• bolt-(sub modules) (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Testing

Verified that the new Javadoc URL returns HTTP 200 OK.

[WilliamBergamin]

Thank you 💯

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.76%. Comparing base (b3e25bf) to head (330f0f5).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1643      +/-   ##
============================================
- Coverage     72.80%   72.76%   -0.04%     
+ Complexity     4551     4548       -3     
============================================
  Files           483      483              
  Lines         14456    14456              
  Branches       1513     1513              
============================================
- Hits          10524    10519       -5     
- Misses         3037     3040       +3     
- Partials        895      897       +2     

Flag	Coverage Δ
jdk-14	72.76% <ø> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

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

[Sandhyasteffy]

Thank you for reviewing and merging my contribution! 😊