Improve the static code analysis tool documentation

[gayaldassanayake]

Description

Currently we have a simple documentaiton on the scan tool in https://ballerina.io/learn/scan-tool/.

However, we need to extend this to include all the essential details for a user including

• Full functionality of the scan tool with the report generation, rule exclusion etc.
• Document the capability of integrating the tool with sonarqube

Version

0.9.0

[gayaldassanayake]

Reopening since we need to add sonar plugin details into the documentation.