Fix broken links on Reactivity Fundamentals page

[ivangermanov]

1. Navigate to the https://vuejs.org/guide/essentials/reactivity-fundamentals.html page.
2. Keep your eyes on the URL and start scrolling down.
3. You will first see https://vuejs.org/guide/essentials/reactivity-fundamentals.html#script-setup (this shouldn't be the case)
4. Then, you will see https://vuejs.org/guide/essentials/reactivity-fundamentals.html#declaring-reactive-state (this should be the case, since it matches the header name).
5. Continue scrolling down where the header is Reactive Proxy vs. Original. The URL there is again https://vuejs.org/guide/essentials/reactivity-fundamentals.html#script-setup, but it should be https://vuejs.org/guide/essentials/reactivity-fundamentals.html#reactive-proxy-vs-original.
6. The same problem with incorrect links appears for the Deep Reactivity (which gets the https://vuejs.org/guide/essentials/reactivity-fundamentals.html#ref-unwrapping-in-reactive-objects) link and Stateful Methods (which gets the https://vuejs.org/guide/essentials/reactivity-fundamentals.html#reactivity-transform link) headers.

This also causes the Algolia search component to not correctly link to the subsections:

I could not find the cause why these links (such as https://vuejs.org/guide/essentials/reactivity-fundamentals.html#ref-unwrapping-in-reactive-objects) exist. I performed a solution-wide search for them, and some of them don't even exist. Others do exist, but have nothing to do with the Reactivity Fundamentals page.

I first discovered this when reading https://vuejs.org/api/composition-api-setup.html#basic-usage and clicking on the automatically shallow unwrapped link in the same section. This links to https://vuejs.org/guide/essentials/reactivity-fundamentals.html#ref-unwrapping-in-templates, but it should probably link to https://vuejs.org/guide/essentials/reactivity-fundamentals.html#deep-reactivity. I'll link a PR to fix that link.

[skirtles-code]

It seems to be a consequence of the API Preference switch. The hidden sections from the other API are being matched.

[NataliaTepluhina]

@ivangermanov I cannot reproduce the described bug anymore 🤔 Please let me know if it's still reproducible for you (if not, let's close the issue)

[ivangermanov]

@NataliaTepluhina It seems like the bug has been resolved one way or another. I couldn't reproduce it again.