feat: [WD-15078] Add skip to content button (#965)

## Done

- Added Skip to content button using the React Components Skip link
component.

Fixes [list issues/bugs if needed]

## QA

1. Run the LXD-UI:
- On the demo server via the link posted by @webteam-app below. This is
only available for PRs created by collaborators of the repo. Ask
@mas-who or @edlerd for access.
- With a local copy of this branch, [build and run as described in the
docs](../CONTRIBUTING.md#setting-up-for-development).
2. Perform the following QA steps:
    -  Start LXD UI, and press tab to reveal the skip link button.

## Screenshots

N/A

src/Root.tsx

@@ -1,6 +1,10 @@
 import { FC } from "react";
 import Navigation from "components/Navigation";
-import { Application, QueuedNotification } from "@canonical/react-components";
+import {
+  Application,
+  QueuedNotification,
+  SkipLink,
+} from "@canonical/react-components";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import { AuthProvider } from "context/auth";
 import { EventQueueProvider } from "context/eventQueue";
@@ -33,6 +37,7 @@ const Root: FC = () => {
                 <OperationsProvider>
                   <EventQueueProvider>
                     <Application id="l-application">
+                      <SkipLink mainId="main-content" />
                       <Navigation />
                       <ErrorBoundary fallback={ErrorPage}>
                         <App />

src/components/BaseLayout.tsx

@@ -17,7 +17,7 @@ const BaseLayout: FC<Props> = ({
   contentClassName,
 }: Props) => {
   return (
-    <AppMain className={mainClassName}>
+    <AppMain className={mainClassName} id="main-content">
       <Panel
         title={title}
         controls={controls}

src/components/CustomLayout.tsx

@@ -15,7 +15,7 @@ const CustomLayout: FC<Props> = ({
   contentClassName,
 }: Props) => {
   return (
-    <AppMain className={mainClassName}>
+    <AppMain className={mainClassName} id="main-content">
       <Panel contentClassName={contentClassName} header={header}>
         {children}
       </Panel>