docs: mention the building variable to execute non-prerenderable code in server handle

[eltigerchino]

closes #9965

Adds a note that the building variable should be used to check that the app isn't building before executing some code only available when not prerendering or building.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13153-svelte.vercel.app/

this is an automated message

[dummdidumm]

Not sure if this suffices - the confusion is also about why handle even is executed

[eltigerchino]

Suggested change

	If you need to exclude some code from running during the build or prerendering step, check that the app is not [`building`]($app-environment#building) beforehand.
	The `handle` hook is invoked each time a route is requested during prerendering. If you need to exclude some code from running during this phase, check that the app is not [`building`]($app-environment#building) beforehand.

Does this preceding sentence help answer that question?

[dummdidumm]

Not quite - the question is also "why is that route even requested during prerendering, I did export const prerender = false"

[eltigerchino]

How about mentioning the crawler? It's true that it crawls both prerendered and non-prerendered pages.

Suggested change

	If you need to exclude some code from running during the build or prerendering step, check that the app is not [`building`]($app-environment#building) beforehand.
	During prerendering, SvelteKit crawls your pages for links and invokes the `handle` hook for each route it finds. If you need to exclude some code from running during this phase, check that the app is not [`building`]($app-environment#building) beforehand.