Reimplement 'slugPreprocessor' for V2

[Simek]

Description

RN website still need this functionality:

• https://react-native-website-migration.netlify.app/docs/button/#nextfocusforward-androidtv
• feat(v1): add 'slugPreprocessor' config option to allow users customize the hash links facebook/docusaurus#3124

[slorber]

This one is a hard one.

Basically we'll need more than "slugPreprocessor", what we want is to have full control and create fixed ids, so that changing text does not break anchor links.

This is particularly important for i18n as translating a md title can lead to broken links on a localized site.

So I think we should find a way to explicitly define the ids instead of having id generation options. This also means that we'll have to check all existing RN website anchor links and apply the fixed id to the target.

Also related:
facebook/docusaurus#3321
facebook/docusaurus#3322

[Simek]

@slorber I think that this issue should not be a blocker for the migration. We have the page live with non-optimal content anchors for few months, before the slugPreprocessor feature. I we can try to fix this after the migration, I can also try to help with some tasks in Docusaurus repo to make it happen quicker 🙂