rename introduction to index for support from select option

[acidiney]

Following the pattern of the previous documentation, I changed the file name introduction.md to index.md, so that I can use the guide/ route, so that from an option in the select of existing versions on the current site, people can follow to version 3 of the documentation.

This PR is linked to this in the current version of the website documentation #2644

[bencodezen]

Hi @acidiney. Thanks for the PR. We're still in the process of working on some information architecture changes, so at this time we're not ready for PRs on this part yet.

To keep track of this, I would definitely like to give you credit for pointing this out by having you create a separate issue regarding a path for those who want to use /guide/ URL.

Appreciate it!