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.

src/pages/about-netbird/other.mdx

@@ -1,6 +1,6 @@
 import {Note} from "@/components/mdx";
 
-export const title = 'Other'
+# Other
 
 ## Google Summer of Code Ideas - 2022
 

src/pages/about-netbird/understanding-nat-and-connectivity.mdx

@@ -1,6 +1,6 @@
 import {Note} from "@/components/mdx";
 
-export const title = 'Understanding NAT and Connectivity'
+# Understanding NAT and Connectivity
 
 ## What is NAT?
 

src/pages/manage/integrations/kubernetes/client-sidecar.mdx

@@ -1,4 +1,4 @@
-## Client Sidecar
+# Client Sidecar
 
 In certain situations we may want to have a pod act like a peer in the Netbird network instead of exposing it through a routing peer. In these cases a Netbird client container has to be added as a sidecar to the pod.