ci: validate PRs with build and MDX heading linter (#743)

* ci: validate PRs with build and MDX heading linter

Adds a pull_request workflow running npm run lint:mdx, npm run build,
and npm run lint so heading-hierarchy bugs and broken builds get caught
before merge rather than after.

The new linter (scripts/lint-mdx-headings.mjs) enforces that the first
heading is h1 and that heading levels never jump by more than one. Also
fixes three existing pages that had no h1 title — two were using a
legacy export const title pattern, one was missing a title entirely.

* ci: use npm install since lockfile is gitignored

package-lock.json is in .gitignore, so npm ci and setup-node's npm
cache both fail on a fresh CI checkout. Match the Dockerfile pattern
(npm install, no cache) instead.

* ci: drop ESLint step; project config is broken

`npm run lint` fails with 'Converting circular structure to JSON'
under ESLint 9.x — the repo has no .eslintrc or eslint.config file,
so the legacy resolver hits the React plugin's circular reference.
This is pre-existing (build_n_push.yml never ran lint, so it stayed
hidden); fixing it needs flat-config migration and is out of scope.
Drop the step until that lands.

package.json

@@ -11,7 +11,8 @@
     "gen": "swagger-codegen generate -i https://raw.githubusercontent.com/netbirdio/netbird/main/management/server/http/api/openapi.yml -l openapi -o generator/openapi && npx ts-node generator/index.ts gen --input generator/openapi/openapi.json --output src/pages/ipa/resources",
     "gen:llm": "node scripts/generate-llm-docs.mjs",
     "start": "next start",
-    "lint": "eslint src/"
+    "lint": "eslint src/",
+    "lint:mdx": "node scripts/lint-mdx-headings.mjs"
   },
   "browserslist": {
     "production": [