docs: add README for module documentation overview

[Vaishnav88sk]

Changelog

• Added a README.md to the docs/docs/concepts/ folder.
• Documented and categorized the purpose of each .md file in the folder.
• Included direct links and descriptions to improve documentation discoverability and onboarding experience.

Additional context (e.g. screenshots, logs, links)

• Improves developer experience by making conceptual documentation easier to navigate.
• Part of enhancing open-source contributions and understanding of the Application SDK.
• Related files: activities.md, application.md, application_sql.md, clients.md, workflows.md, and others in the concepts folder.

Checklist

• Additional tests added
• All CI checks passed
• Relevant documentation updated

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Copyleft License Compliance

• Have you used any code that is subject to a Copyleft license (e.g., GPL, AGPL, LGPL)?
• If yes, have you modified the code in the context of this project? please share additional details.

[inishchith]

@Vaishnav88sk - we no longer maintain docs in the repository itself. we are revamping the documentation space, I'll go ahead and close this PR. Thanks for contributing.