doc: add contributor quickstart and improve README contributing section

[MikeGarciaAGM]

Description

This PR improves contributor onboarding docs with a concise quickstart focused on small, review-friendly documentation pull requests.

Changes:

• Added a dedicated quickstart guide for contributors.
• Improved README contributing section wording.
• Linked README contributing section to the new quickstart for easier navigation.

This is docs-only and scoped to be simple to review.

Notes

• No code/runtime changes.
• No dependency changes.
• Intended to reduce onboarding friction for first-time contributors.

BTC/BGL PR reward address

(leave blank if not required by maintainers)

[MikeGarciaAGM]

Quick follow-up: this PR is ready for review and merge from my side. Happy to apply any requested changes immediately.

[MikeGarciaAGM]

Quick follow-up on this bounty PR: all requested docs updates are in and the diff is still small/clean.

PR: #325
Issue: #32

If any wording or structure should be adjusted to meet merge standards, I can patch it immediately.

[MikeGarciaAGM]

Quick follow-up: this PR is still ready for review/merge on my side. Happy to adjust immediately if maintainers want any changes.

[MikeGarciaAGM]

Friendly follow-up: this PR is ready for review and merge (mergeable: clean). It only updates docs with bounty list + direct links and includes the requested demo notes. Thanks for taking a look when possible.

[MikeGarciaAGM]

Quick follow-up from claimant side: PR #325 is still mergeable-clean on latest check and I can make any maintainer-requested edits immediately. If current format is payout-eligible as-is, I am ready for final review/merge.

[MikeGarciaAGM]

Quick follow-up: this PR is ready for review/merge from my side. I can apply any requested edits quickly if maintainers want adjustments. Thanks!