Document Plugins addition

[Zaxounette]

Document Plugins addition once feature is out.
BTCPayServer#2006 for reference.

• Mention it as an experimental feature (for the time being) in relevant docs (root readme.md as an example)
• Create specific doc for more detailed explanation.
  Possibly split info between doc relevant to users and doc relevant to devs looking to create plugins.
• Sample code and blueprint plugin for devs (BTCPay E̴x̴t̴e̴n̴s̴i̴o̴n̴s̴ Plugins btcpayserver#2006 (comment)

[Bas02]

i think we can start to clean this up as btcpayserver/btcpayserver#3862 being worked on.

Also including this : https://docs.btcpayserver.org/Walkthrough/#plugins as it's the only place currently.
Probably good to pour this into a dedicated section :)

@pavlenex can you include this in roadmap and possibly for 1.6.0 ?

[pavlenex]

Dev docs for this exist for a while, https://docs.btcpayserver.org/Development/Plugins/#setup-of-a-new-plugin probably should be added as a feature in the user docs as well.