Svelte: Fix hash links in docs

[Rich-Harris]

Some places in the docs link to other places in the docs. Those links are now out of date because the slugs have changed

[vovs03]

:beetle: Broken links

I has parse all site by Screaming Frog SEO Spider and finded this:

Fixing

• [ ] http://svelte.dev/TODO-blog-post-on-css-in-js
• [ ] https://svelte.dev/TODO-blog-post-on-css-in-js
• [ ] http://svelte.dev/the-zen-of-just-writing-css
• [ ] http://svelte.dev/api/blog/sapper-towards-the-ideal-web-app-framework
• [ ] http://svelte.dev/tutorials/onmount
• [ ] https://svelte.dev/the-zen-of-just-writing-css
• [ ] https://svelte.dev/api/blog/sapper-towards-the-ideal-web-app-framework
• [ ] https://svelte.dev/guide
• [ ] https://svelte.dev/tutorials/onmount

[Conduitry]

I _think_ we're good here. We can address other broken links as they surface.