chore: replace concurrently with npm-run-all in docs and design-system (#47697)

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

Chore / dependency cleanup.

## What is the current behavior?

`apps/docs` and `apps/design-system` use the `concurrently` package to
run their parallel dev scripts, while the rest of the monorepo (e.g.
`packages/pg-meta`) already relies on `npm-run-all`.

## What is the new behavior?

Both apps now use `npm-run-all` (`run-p`), and `concurrently` is dropped
from their dependencies and the lockfile; `apps/docs` keeps its
kill-on-exit behavior via `run-p --race`, and `apps/design-system`'s
`dev:full` is collapsed into `dev` using the `run-p dev:*` glob (with
`dev:next`/`dev:content` sub-tasks and README updated to match).

## Additional context

N/A

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Updated design system setup instructions to reflect the new
development workflow and MDX watching behavior.
* **Chores**
* Simplified local development scripts for the design system and docs
apps.
* Split the dev workflow into separate commands for the app server and
content watcher, making it easier to run and troubleshoot.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/docs/package.json

@@ -22,7 +22,8 @@
     "codegen:references:legacy": "tsx features/docs/Reference.generated.script.ts",
     "codegen:references": "pnpm codegen:references:legacy && pnpm codegen:references:new",
     "codemod:frontmatter": "node ./scripts/codemod/mdx-meta.mjs && prettier --cache --write \"content/**/*.mdx\"",
-    "dev": "concurrently --kill-others \"next dev --port 3001\" \"pnpm run dev:watch:troubleshooting\"",
+    "dev": "run-p --race dev:next dev:watch:troubleshooting",
+    "dev:next": "next dev --port 3001",
     "dev:secrets:pull": "AWS_PROFILE=supa-dev node ../../scripts/getSecrets.js -n local/docs",
     "dev:watch:troubleshooting": "node ./scripts/troubleshooting/watch.mjs",
     "embeddings": "tsx --conditions=react-server scripts/search/generate-embeddings.ts",
@@ -162,12 +163,12 @@
     "api-types": "workspace:*",
     "cheerio": "^1.0.0-rc.12",
     "chokidar": "^4.0.3",
-    "concurrently": "^9.1.2",
     "config": "workspace:*",
     "dotenv": "^16.0.3",
     "ejs": "^3.1.10",
     "globby": "^13.2.2",
     "graphiql": "^4.0.2",
+    "npm-run-all": "^4.1.5",
     "openapi-types": "^12.1.3",
     "postcss": "catalog:",
     "shiki": "^3.2.1",