ipv6-staging explained (#341)

Co-authored-by: sanderjo <sander.jonkers+github@github.com>

wiki/advanced/ipv6-staging.md

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+# IPv6 Staging
+
+With ipv6-staging, you can activate special IPv6 features in SABnzbd that are not yet mainstream.
+
+You can find ipv6-staging in Config -> Special.
+
+## Features
+
+### Enrich newsservers with IPv6 connectivity.
+
+Some usenet providers have an IPv4-only newsserver, and a separate IPv6-only newsserver. 
+To be more exact: it's probably the same newsserver, but there is an IPv4-ony name (FQDN), and an IPV6-only.
+For example: news.eweka.nl and news6.eweka.nl. 
+
+These providers want the user to specify this separate newsserver by hand. 
+However, this is more work for the user (a separate entry in the Server list), and it's annoying if the user has a laptop which sometimes has and sometimes hasn't got IPv6-ocnnectivity.
+
+SABnzbd solves this with this ipv6-staging feature: If you turn it on, SABnzbd will enrich 
+the IPv4-only name with the IPv6-only name (for well-known nesservers).
+Combined with HappyEyeballs, SABnzbd will find the connection that works quickest.
+This is fail-safe: if there is no IPv6, or it's slow, SABnzbd will use IPv4.
+
+What is the advantage of using IPv6? With IPv6, you have no NAT (and no CGNAT), so the IPv6 connection might be faster.
+
+You can turn on this feature even if you haven't (yet) IPv6.
+
+Example logging in sabnzbd.log (with +Debug turned on):
+
+```
+2024-10-29 11:37:07,682::DEBUG::[downloader:218] Retrieving server address information for <Server: id=-1, host=news.eweka.nl:563>
+2024-10-29 11:37:07,682::INFO::[happyeyeballs:136] Added alternative IPv6 address: news6.eweka.nl
+2024-10-29 11:37:07,710::DEBUG::[happyeyeballs:168] Available addresses for news.eweka.nl (port=563, IPv4 or IPv6): 4 IPv4 and 3 IPv6
+2024-10-29 11:37:07,719::DEBUG::[happyeyeballs:100] Happy Eyeballs connected to 2001:4de0:1::233 (news6.eweka.nl) in 8ms
+2024-10-29 11:37:07,719::INFO::[happyeyeballs:201] Quickest IP address for news.eweka.nl (port=563, IPv4 or IPv6): 2001:4de0:1::233 (news6.eweka.nl)
+2024-10-29 11:37:07,776::INFO::[newswrapper:336] -1@news.eweka.nl: Connected using TLSv1.3 (TLS_AES_256_GCM_SHA384)
+```
+
+