Added high-level README files

[sujaykarmakar]

Summary

This PR adds package-level README.md files for the following core Tomcat packages:

• java/org/apache/catalina/README.md
• java/org/apache/coyote/README.md
• java/org/apache/tomcat/util/README.md

What’s included

Each README provides a high-level overview of the package, including:

• The package's purpose and responsibilities
• Major components and commonly used classes/interfaces
• How the package fits into Tomcat's overall architecture
• Relationships with other core packages

Motivation

While Tomcat has comprehensive Javadocs and architecture documentation, it can take some time for new contributors to understand the role of each package when navigating the source tree. These READMEs are intended to serve as quick entry points and improve codebase discoverability without duplicating existing documentation.

Impact

• Documentation-only change
• No code, API, or behavioral changes
• No impact on build, tests, or runtime

[markt-asf]

That is what package level Javadoc is for.