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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🔨 Latest commit	666ec39
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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 90.21%. Comparing base (fd1c648) to head (666ec39).

Additional details and impacted files

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##             main    #1528   +/-   ##
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  Coverage   90.21%   90.21%           
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  Files          69       69           
  Lines        5511     5511           
  Branches      944      944           
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  Hits         4972     4972           
  Misses        521      521           
  Partials       18       18           

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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 👍🏼

[jescalada]

Added link to the Processors.md guide for more details:

Suggested change

	GitProxy applies a series of automated checks (called processors) to every push. These checks run in sequence, and if any check fails, the push is blocked. The main checks include:
	GitProxy applies a series of automated checks (called [Processors](https://github.com/finos/git-proxy/blob/main/docs/Processors.md)) to every push. These checks run in sequence, and if any check fails, the push is blocked. The main checks include:

[jescalada]

I'm wondering if we could specify the expected error messages here, similar to the troubleshooting guide for the v2 upgrade.

It'll help users identify their problem faster, and also improve the likelihood of finding this doc page when they (or their AI agents) Google the error message.