docs: add user manual for end-user contribution workflow

[kriswest]

resolves #1525

Summary

Adds a new website/docs/user-manual.mdx page providing a comprehensive guide for end users contributing to projects through a GitProxy deployment.

Preview deeplink: https://deploy-preview-1528.git-proxy.preview.finos.org/docs/user-manual

Motivation

The existing documentation covers installation, configuration, and development but lacks a single end-to-end guide for the primary audience: developers pushing code through GitProxy. This page fills that gap.

What's included

• Introduction explaining GitProxy's purpose and supported destinations
• Mermaid sequence diagrams illustrating ingress and egress flows
• Authentication guidance covering both proxy (PAT) and UI credentials
• Overview of automated checks (with a note on plugin extensibility)
• Instructions for setting up forks, configuring the git client, cloning, pushing, requesting review, and pushing approved changes
• Reviewer workflow
• Troubleshooting section covering common issues (credentials, blocked pushes, SSL errors, DCO sign-offs)

Design decisions

• Written to be organisation-agnostic — uses :::info admonitions wherever deploying organisations' specific configuration will affect the answer (authentication method, checks applied, access provisioning).
• Uses Mermaid diagrams (rendered natively by Docusaurus) rather than static images.
• References existing docs (authentication, plugins, deployment) rather than duplicating content.

[netlify]

✅ Deploy Preview for endearing-brigadeiros-63f9d0 ready!

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[github-actions]

Dependency Review

The following issues were found:

• ✅ 0 vulnerable package(s)
• ✅ 0 package(s) with incompatible licenses
• ✅ 0 package(s) with invalid SPDX license definitions
• ⚠️ 1 package(s) with unknown licenses.
• ⚠️ 42 packages with OpenSSF Scorecard issues.

View full job summary

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (faeb58b) to head (0d6052c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1528   +/-   ##
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  Coverage   90.25%   90.25%           
=======================================
  Files          69       69           
  Lines        5561     5561           
  Branches      958      958           
=======================================
  Hits         5019     5019           
  Misses        523      523           
  Partials       19       19           

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[jescalada]

Looks good to me! Just wondering if we can add more details to the troubleshooting guide, mostly for SEO and UX reasons 👍🏼

[kriswest]

Updated, adding link to processors.md and an info box about git auth popups.

I think this is good to go for now. Quoting specific error messages etc. could be a later refinement - perhaps worth addressing alongside revised UI and adding some screenshots @jescalada

[jescalada]

LGTM! You're right, a detailed troubleshooting guide would make more sense with the UI updates merged in - screenshots are much more useful than plain messages. That said, keeping the error messages alongside the screenshots is best for SEO and UX.