Add frontend validation checks and conditional build steps in workflows

[javidahmed64592]

This pull request introduces conditional frontend validation and build steps to the CI and build workflows, making them more flexible for repositories that may not include a frontend directory. It adds a new action to check for the existence of the frontend directory and updates documentation and workflow files to reflect these changes. The workflows now skip frontend jobs if the directory is not present, and the documentation provides clear guidance on these enhancements.

Workflow enhancements for conditional frontend handling:

• Added a new check-frontend-exists action (.github/actions/setup/check-frontend-exists/action.yml) that checks for the presence of the <repository name>-frontend directory and outputs an existence flag. This enables conditional execution of frontend-related jobs. [1] [2]
• Updated CI (ci.yml) and build (build.yml) workflows to use the new check and conditionally run frontend validation and build steps only if the frontend directory exists. This includes the new frontend job in CI and build-frontend job in build, with downstream jobs depending on the existence flag. [1] [2]

Documentation updates:

• Expanded WORKFLOWS.md to document the new check-frontend-exists action, its outputs, and usage, as well as the conditional logic in CI and build workflows. [1] [2]
• Updated descriptions and usage instructions for the frontend and build-frontend actions to clarify their new roles and correct repository references. [1] [2] [3] [4]

Minor workflow improvements:

• Added job dependencies and artifact handling for frontend builds in the build workflow, ensuring artifacts are downloaded when present.
• Updated CI jobs to reference actions from the correct repository and improved job descriptions for clarity. [1] [2] [3] [4] [5] [6] [7]