Fix search box on alert and tooltip pages

What does this MR do?

Fix alert page search box

In production only, if you visit the /components/alert page directly, the sidebar's search box doesn't render.

In the console, there's this error:

DOMException: Node.appendChild: Cannot add children to a Text
  > NuxtJS

This fix is bizarre.

Here are a couple of patches that fix the problem:

• Remove HTML syntax highlighting on a code block.

  diff --git c/contents/components/alert.md i/contents/components/alert.md
  index f16c011b90..8aa2c65c02 100644
  --- c/contents/components/alert.md
  +++ i/contents/components/alert.md
  @@ -117,7 +117,7 @@ Alerts don't handle their own visibility, so it's the parent component's
   responsbility to listen for the `dismiss` event and hide the alert in some way.
   For example:
  
  -```html
  +```
   <script>
     ...
     computed: {
  

• Don't do property access at the beginning of an expression?

  diff --git c/contents/components/alert.md w/contents/components/alert.md
  index 0697bc33f9..86155ad257 100644
  --- c/contents/components/alert.md
  +++ w/contents/components/alert.md
  @@ -124,7 +124,7 @@ For example:
     ...
     computed: {
       shouldShowAlert() {
  -      return !this.isAlertDismissed && this.someOtherCondition();
  +      return !isAlertDismissed && this.someOtherCondition();
       },
     },
     ...

It appears that a minimal reproduction is to have a page with this in it:

```html
<script>a.b</script>
```

Figuring out why that breaks anything at all probably means digging into the guts of Nuxt. For now, we'll work around it.

Addresses #2953 (closed).

Fix tooltip page search box

Much like the previous commit, this fix is bizarre.

This time, the problem isn't property access at the beginning of an expression in a script tag of an HTML block, but:

• An import statement.
• A key/value pair in an object.

Both of these need to be removed to fix it:

diff --git c/contents/components/tooltip.md i/contents/components/tooltip.md
index 1098987200..5cabb038cc 100644
--- c/contents/components/tooltip.md
+++ i/contents/components/tooltip.md
@@ -134,11 +134,8 @@ You will need to import and register `GlTooltipDirective` before you can use it.

 ```html
 <script>
-import { GlTooltipDirective } from '@gitlab/ui';
-
 export default {
   directives: {
-    GlTooltip: GlTooltipDirective,
   },
 };
 </script>

Instead, we mark it as plan text to work around the issue.

Addresses #2953 (closed).

Validating locally

1. yarn build
2. python3 -m http.server 3000 --bind localhost --directory public
3. Visit http://localhost:3000/components/alert
4. Observe search box renders
5. Visit http://localhost:3000/components/tooltip
6. Observe search box renders

Integrations

• GitLab: mr_url
• CustomersDot: mr_url
• Duo UI: mr_url
• Status Page: mr_url
• Docs: mr_url
• Switchboard: mr_url

Does this MR meet the acceptance criteria?

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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Related to #2953 (closed)