add react & js docs for feature flag setup

[annabfenske]

What does this PR do? What is the motivation?

Adds setup instructions for feature flags JavaScript & React SDKs.

Merge instructions

Merge readiness:

• [ ] Ready for merge

For Datadog employees:

Your branch name MUST follow the <name>/<description> convention and include the forward slash (/). Without this format, your pull request will not pass CI, the GitLab pipeline will not run, and you won't get a branch preview. Getting a branch preview makes it easier for us to check any issues with your PR, such as broken links.

If your branch doesn't follow this format, rename it or create a new branch and PR.

[6/5/2025] Merge queue has been disabled on the documentation repo. If you have write access to the repo, the PR has been reviewed by a Documentation team member, and all of the required checks have passed, you can use the Squash and Merge button to merge the PR. If you don't have write access, or you need help, reach out in the #documentation channel in Slack.

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/annabfenske/feature-flags-js-react-docs/feature_flags/client/javascript
• https://docs-staging.datadoghq.com/annabfenske/feature-flags-js-react-docs/feature_flags/client/react

[buraizu]

Created DOCS-12884 for documentation team review