CI: Manifest workflows + daily schedule + optional S3 upload

[sfc-gh-kkomail]

Adds manifest artifact/publish workflows, daily 11:59 PM PT schedule, and optional S3 upload (env-driven). No guide content format changes; manifest JSON structure is unchanged.

[kameshsampath]

• Lets avoid using repo secrets and use Web Identity. An example workflow https://gist.github.com/kameshsampath/afdf41f517f6ceb190db6fc87c71a423

• I think you can keep the manifest_generator.py just to generate the manifest JSON and aws cli should be in path already to do the upload so use aws s3 ...

• We can merge manifest-artifact.yml and manifest-publish-branch.yml as two jobs within same workflow ? Since the upload depends on the mainfest generation, its better it to run after the generate than in parallel

• Nice to see adding you are adding that artifact as build run asset. Its very helpful

• Are we still committing to the repo or sticking just to upload to s3?

[kameshsampath]

why do we need two jobs.github/workflows/manifest-artifact.yml and .github/workflows/manifest-publish-branch.yml ? guess one of them is stale ? Also lets rename the manifest-publish-branch.yml