Improve Email follow-ups configuration and fix missing logo

[gonzaleztroyano]

• Fix some lists, the missing linebreak was causing Sphinx treating it as a paragraph.
• Use a table to improve readability for global configuration options.
• Add additional info about IMAP OAth (internal link)
• Add missing GLPI logo. It's now shown on top of sidebar.
• Minor fixes

Build preview:

[cconard96]

Looks good to me, except maybe the logo (blue logo on blue background). The white version of this logo may look better here (pics/logos/logo-GLPI-25-white.png in the GLPI files).

[gonzaleztroyano]

Hi, @cconard96! Thanks for your feedback. I've updated the image.