[Improvement]: Add README.md as the primary documentation file for library packages

[azinneera]

Description

Currently, the documentation is captured in two files: Package.md for package documentation and additionally, a Module.md for module-specific documentation. This has to be improved to reduce to one file, and since most of the other platforms like GitHub supports README.md by default for documentation, we should use the same name.

Moreover, the user must have the option to specify a custom .md file for documentation. A new field under the [package] table in the Ballerina.toml file must be introduced for this.

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-> Compilation

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