docs: document Zero --security token and whitelist flags - #762

Cuts the v25.4 documentation version and promotes it to the version
served at the site root, matching the
[v25.4.0](https://github.com/dgraph-io/dgraph/releases/tag/v25.4.0)
release (2026-07-30).

## Summary

- Snapshots `docs/` into `docs_versioned_docs/version-v25.4/` and
`docs-graphql/` into `graphql_versioned_docs/version-v25.4/`, via
`docs:version:docs` and `docs:version:graphql`.
- Sets `lastVersion: 'v25.4'` on both content plugins, with v25.4 at
path `''` and v25.3 moved to `v25.3`.
- The snapshot carries the Zero `--security` documentation merged in
#762, the one user-facing change in v25.4.0.

## Also in here: a graphql version-map repair

Worth a look, since it is easy to miss in a 227-file diff. The v25.3 cut
(b75fd1a) added `"v25.3"` to `graphql_versions.json` but never added a
matching entry to the graphql plugin's `versions` map in
`docusaurus.config.ts`. Docusaurus fell back to defaults for it, so
`/graphql/` kept serving **v25.2 (latest)** while v25.3 sat above it in
the dropdown as an older version. Both v25.3 and v25.4 now have explicit
entries, and the GraphQL section moves from v25.2 to v25.4 as latest.

## URL movement

Normal for any cut, but the GraphQL section advances two versions at
once:

| Path | Before | After |
|------|--------|-------|
| `/<page>` | v25.3 | v25.4 |
| `/v25.3/<page>` | 404 | v25.3 |
| `/graphql/<page>` | v25.2 | v25.4 |
| `/graphql/v25.2/<page>` | 404 | v25.2 |
| `/graphql/v25.3/<page>` | v25.3 | v25.3 (unchanged) |

## Testing

`npm run build` passes. With `onBrokenLinks: 'throw'`, a clean build is
the link check across all five versions. Verified against the generated
output in `build/`:

- `/` and `/graphql/` both render **v25.4 (latest)**, with v25.3, v25.2,
v25.1, and v24.1 in the dropdown.
- `/cli/zero` and `/admin/security/admin-endpoint-security` include the
`--security` content; `/v25.3/` equivalents do not, since the flag is in
no v25.3.x binary.
- All eight older-version paths resolve.

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>