Add some more content to architecture documentation

[adammurdoch]

Context

This PR fleshes out the "entry point" for the Gradle architecture to discuss a few important concepts in the Gradle internals.

Contributor Checklist

• Review Contribution Guidelines.
• Make sure that all commits are signed off to indicate that you agree to the terms of Developer Certificate of Origin.
• Make sure all contributed code can be distributed under the terms of the Apache License 2.0, e.g. the code was written by yourself or the original code is licensed under a license compatible to Apache License 2.0.
• Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by Gradle team.
• Provide integration tests (under <subproject>/src/integTest) to verify changes from a user perspective.
• Provide unit tests (under <subproject>/src/test) to verify logic.
• Update User Guide, DSL Reference, and Javadoc for public-facing changes.
• Ensure that tests pass sanity check: ./gradlew sanityCheck.
• Ensure that tests pass locally: ./gradlew <changed-subproject>:quickTest.

Reviewing cheatsheet

Before merging the PR, comments starting with

• ❌ ❓must be fixed
• 🤔 💅 should be fixed
• 💭 may be fixed
• 🎉 celebrate happy things

[eskatos]

👍