Improve searchability of OIDC well known providers guide

[sberyozkin]

Description

When we go to Quarkus guides, it is hard to find and hence believe that authenticating to most well-known social providers is a very simple straightforward process.
Authenticating to providers like Apple is possibly the simplest of its kind around.
The existing OIDC guide documenting it all is unfortunately very hard to find, was a news to everyone in the CC list 🙂

Implementation ideas

Create a tag per every supported social provider and associate it with this guide, verify that both Google and Quarkus Guides search experience has improved.
Also add a link to this doc from a top level OIDC into section in the authentication mechanisms guide.

Will appreciate help from the Doc team 🙏

CC @maxandersen @cescoffier @stuartwdouglas @FroMage @n1hility

[quarkus-bot]

/cc @pedroigor (oidc)