Improve `Client Migration to Keycloak` instructions

[nikosev]

Summary

This PR introduces the following changes:

• Break Endpoints table per environment using tabpanex
• Add section for the intrspection endpoint in migration guide
• Add "Common Issues" section
• Add sub-sections in Client Migration to Keycloak
• Update endpoints with the Keycloak OIDC Endpoints in OpenStack guide

[github-actions[bot]]

Documentation preview deployed!

Available at https://docs.egi.eu/documentation/461

[gwarf]

Thanks, I feel the tapanex is easier to read. Could you pleaes also replace the table at the end of https://docs.egi.eu/documentation/461/providers/check-in/sp/#general-information ? Same for info on https://docs.egi.eu/documentation/461/providers/check-in/idp/ ?

[github-actions[bot]]

Documentation preview deployed!

Available at https://docs.egi.eu/documentation/461

[gwarf]

Duh, the link check are successful, the pages disappeared: https://github.com/EGI-Federation/documentation/runs/7007222970?check_suite_focus=true#step:4:43

[gwarf]

Thanks @nikosev , apparently now there is mainly a table left at https://docs.egi.eu/documentation/461/providers/check-in/sp/#metadata-registration

[github-actions[bot]]

Documentation preview deployed!

Available at https://docs.egi.eu/documentation/461

[gwarf]

Thanks, it looks good. For me the last point to address is the broken link: https://github.com/EGI-Federation/documentation/runs/7019213952?check_suite_focus=true#step:4:43 https://www.geant.org/Services/Trust_identity_and_security/Pages/NamespaceRegistry.aspx no more works, @nikosev , @NicolasLiampotis do you have a replacement or updated link for this?

[github-actions[bot]]

Documentation preview deployed!

Available at https://docs.egi.eu/documentation/461