sync_windows10.rst: Cleanup and document TLSv1.2

[bdrewery]

• Remove redundant Contacts section. The remaining section works and is simpler.
• Remove redundant "repeat steps" step
• Move credit to the bottom as it is just a distraction
• Document the server-side TLSv1.2 requirement for Windows 10's schannel TLS client support.

The TLSv1.2 requirement is the primary change I want to get added here. The rest is not strictly needed.

Keywords for the sake of SEO to help others find this: Windows 10 calendar contacts nextcloud owncloud caldav carddav can't get calendar fix account does not work sync

[pierreozoux]

@bdrewery could you resolve conflicts please? Danke!