Introduce an advanced topics section to the ballerina website

[gayaldassanayake]

Description

Certain advanced topics are not required for any beginner or intermediate users, but are helpful for the advanced users.

Following are such examples.

• Project API
• Compiler Plugins
• Bal tool creation
• Tool integration with package build
• Creating custom rules for bal scan tool

For some of these cases,

• Project API - we do not have documentation at all
• Compiler plugins - Articles written by internal members
  • https://medium.com/ballerina-techblog/module-import-restriction-with-ballerina-code-analyzers-5e21d9644985
• Bal tool/ tool integration with package build - internal documentation in google docs
  • [Developer Guide] Tool Integration with Package Build
  • [Developer Guide] Bal Tool
  • Static Code Analyzer Compiler Plugin

Related website/documentation area

Add/Uncomment the relevant area label out of the following.

Describe the problem(s)

Not having official documentation can cause time-wasting and confusion for internal developers. External developers may not be able to find these unofficial how-to guides, hence might not be able to accomplish desired tasks entirely.

Describe your solution(s)

Introduce a section of documentation for advanced users. For this, we need to look at how other programming languages/ technologies have documented and presented such topics to the users.

Related issue(s) (optional)

Any related issues such as sub tasks and issues reported in other repositories (e.g., component repositories), similar problems, etc.

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[gayaldassanayake]

Had an offline discussion regarding this with @anupama-pathirage. All these topics are mostly used by internal developers. As an initial step, these documentations will be added under https://github.com/ballerina-platform/ballerina-lang/tree/master/docs.

Related design doc - https://docs.google.com/document/d/145VFCbfUZxB6HZ3RHKgHEcKehXqRvFEZHNqeG--tYt8/edit?usp=sharing