feat(a11y): Add skip-to-content accessibility link (#17527)

## DESCRIBE YOUR PR
Adds a root-level skip-to-content link and ensures non-DocPage routes
have a matching `#main` target. This keeps keyboard users from getting
stuck on the home page or 404 page when using the skip link.

This replaces fork PR #17479 so the Codeowner assignment workflow can
run with same-repo permissions.

## IS YOUR CHANGE URGENT?  

- [ ] Urgent deadline (GA date, etc.):
- [ ] Other deadline:
- [x] None: Not urgent, can wait up to 1 week+

## SLA

- Teamwork makes the dream work, so please add a reviewer to your PRs.
- Please give the docs team up to 1 week to review your PR unless you
have added an urgent due date to it.
Thanks in advance for your help!

## PRE-MERGE CHECKLIST

- [ ] Checked Vercel preview for correctness, including links
- [x] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

---------

Co-authored-by: macuzi <lamberto.asghedom@sentry.io>

Author: 2 people

app/globals.css

@@ -4,6 +4,28 @@
 @tailwind components;
 @tailwind utilities;
 
+.skip-to-content {
+  position: fixed;
+  top: 0.75rem;
+  left: 0.75rem;
+  z-index: 9999;
+  padding: 0.5rem 1rem;
+  border-radius: 0.5rem;
+  background-color: var(--accent-purple);
+  color: #fff;
+  font-weight: 500;
+  text-decoration: none;
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.2);
+  transform: translateY(-200%);
+  transition: transform 0.15s ease-in-out;
+}
+
+.skip-to-content:focus {
+  transform: translateY(0);
+  outline: 2px solid var(--accent-purple);
+  outline-offset: 2px;
+}
+
 .radix-themes {
   --cursor-button: pointer;
   --iris-9: var(--accent);

app/layout.tsx

@@ -5,6 +5,7 @@ import type {Metadata} from 'next';
 import {Rubik} from 'next/font/google';
 import Script from 'next/script';
 import PlausibleProvider from 'next-plausible';
+import {SkipToContent} from 'sentry-docs/components/skipToContent';
 import {ThemeProvider} from 'sentry-docs/components/theme-provider';
 
 const rubik = Rubik({
@@ -43,6 +44,7 @@ export default function RootLayout({children}: {children: React.ReactNode}) {
           disableTransitionOnChange
         >
           <Theme accentColor="iris" grayColor="sand" radius="large" scaling="95%">
+            <SkipToContent />
             {children}
           </Theme>
         </ThemeProvider>

app/not-found.tsx

@@ -45,7 +45,7 @@ export default function NotFound() {
       ) : (
         <Header pathname="/" searchPlatforms={[]} noSearch platforms={[]} />
       )}
-      <main className="px-8 pt-28">
+      <main id="main" className="px-8 pt-28">
         <h1 className="font-medium text-3xl mb-4">Page Not Found</h1>
         <p className="text-lg">We couldn't find the page you were looking for :(</p>