Document onboarding process #3442 issue solved

[paras-chinchalkar]

Title: docs: Add onboarding guide and README link

Body:
Short summary:
Adds a concise onboarding guide for new Augur contributors and links it from the repository README to help newcomers find community, setup, and first-contribution resources more quickly.

What changed:

Added docs/onboarding.md — a short, actionable onboarding guide covering:
joining the CHAOSS community (Slack, meetings, YouTube)
running Augur locally (docs + Docker pointers)
finding starter issues (first-timers-only, good first issue)
where to ask for help and how to escalate
pointers for more challenging work
Updated README.md — inserted an "Onboarding for New Contributors" section linking to docs/onboarding.md.
Why:
This provides a clear, discoverable path for newcomers (derived from community suggestions) to get started contributing to Augur and reduces friction for first-time contributors. Relates to community discussion in issues #3442 and #3446.

Related issues / credit:

References: #3442, #3446
Contributor: @MoralCode (original suggestion)
Files: docs/onboarding.md, README.md

[sgoggins]

@paras-chinchalkar : I am unfamiliar with the onboarding.md file's origin story. I know there are an ever growing number of "standard" files being dropped into repositories, but I could use a little history and context to evaluate whether this file fits how we operate. Also interested in what @MoralCode thinks.

THANK YOU for the contribution. This comment is simply to make you aware that we need a minute. :)

[MoralCode]

I have already given this feedback to another contributor who is solving this same issue, but IMO this list of guideposts (from the issue) should be in CONTRIBUTING.md. Im not aware of ONBOARDING.md being a widely used practice

[MoralCode]

Please move this content to CONTRIBUTING.md. Your changes in #3448 (comment) may also be a helpful summary too if you decide to bring those over.