docs: document incoming webhooks for Flipt v2 Pro

[markphelps]

Document incoming SCM webhooks for Flipt v2 Pro so users can fetch flag changes after a push or merge without waiting for the next poll.

• Add a Webhooks page under Configuration with a NEW badge, covering configuration, provider authentication, response codes, metrics, and polling fallback.
• Add an operations guide for GitHub, GitLab, Gitea, Bitbucket, and Azure DevOps, including delivery testing, troubleshooting, and replica behavior.
• Document environments.[id].scm.webhook.secret in the general configuration reference and link the feature from the Pro, environment, Git sync, and SCM pages.

Verified behavior against flipt-io/flipt#6643. The docs explain that 202 means a fetch was scheduled and that the endpoint path uses the configured environment name.

Validation: ESLint, Prettier, Vale, Mintlify build validation, and broken-link checks pass. YAML examples match Flipt's configuration schema. Checked all changed pages and provider tabs in the local browser preview. Independent review findings were addressed. Live provider deliveries were not tested.

Closes #425.

[github-actions]

Verdict: comment

Clean, well-verified docs addition for Flipt v2 Pro incoming webhooks. Both new pages have required frontmatter, code blocks are language-tagged, Mintlify components are valid, and all internal anchors and external links resolve and use markdown syntax. Version placement (v2, Pro-labeled) and the ${env:...}/${secret:...} config syntax match existing conventions. One unexplained navigation edit in docs.json is worth confirming.

docs/docs.json

• minor (L45): This PR removes the tag: "NEW" from the Pro Features nav group, but that change isn't mentioned in the PR description and is unrelated to the webhooks feature (which gets its own page-level tag: NEW in v2/webhooks/overview.mdx). Removing the group-level tag drops the NEW badge from the entire Pro Features group in the sidebar, not just webhooks. If this is intentional cleanup (the group is no longer "new"), consider noting it in the PR body; if accidental, restore the tag: "NEW" line.

🤖 Automated review by the Flipt PR review agent.