Uplinks docs should explain per-uplink proxy support.

[mk-pmb]

Thank you for this great project!

Is your feature request related to a problem?
I'm uncertain whether I can configure my VPN proxy in verdaccio's config or whether I'll need to plug another proxy in between.

Describe the solution you'd like
https://verdaccio.org/docs/en/uplinks should

• explain whether and how it is possible to set a parent HTTP proxy specifically for one uplink. (My current guess: Not possible yet.)
• explain whether and how I can enforce using the HTTP CONNECT method even if the registry URL uses plain HTTP. (To circumvent a bug in the parent proxy)
• explain whether and how I can limit how many connections to that proxy verdaccio will try to open at the same time. (Concerned because npm v6 set a very bad example at that, and would abort the entire install attempt when its parent HTTP proxy refuses additional connections.)
• remind readers that https://verdaccio.org/docs/en/configuration has options for a default proxy.

Describe alternatives you've considered

Additional context
If I'm correct in assuming that the lack of documentation means lack of feature, prepare for the related feature requests.