Update docs to show how load differs from Next.js's getStaticProps

[Nick-Mazuk]

Currently the docs say that the load function is the Sveltekit equivalent to Next.js's getStaticProps or getServerSideProps.

load is the SvelteKit equivalent of getStaticProps or getServerSideProps in Next.js or asyncData in Nuxt.js.

This PR reflects the fact that Sveltekit's load function also runs on the client. Here's the new sentence:

load is similar to Next.js's getStaticProps or getServerSideProps or Nuxt.js's asyncData, except that it runs on both the server and the client.

In Next.js, getStaticProps and getServerSideProps both only run on the server. This means it's safe to run privileged code in these functions. However, according the current Sveltekit docs, load runs on both the client and server, meaning that you should not run privileged code in it.

This PR should help prevent Sveltekit users from accidentally creating security vulnerabilities in their apps due to this difference.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts

[Rich-Harris]

thanks!