docs: add gateway documentation for commands, configuration, and local development

[aidandaly24]

Description

Adds documentation for the gateway integration feature across existing and new docs:

• docs/gateway.md (new) — Gateway overview, quick start, targets, inbound/outbound authentication, adding gateways to existing projects, and local development
• docs/commands.md — Added add gateway, add gateway-target commands; updated add identity to include OAuth credential type; updated remove with gateway/gateway-target examples
• docs/configuration.md — Added mcp.json schema reference (gateways, targets, CUSTOM_JWT authorizer, outbound auth); updated credential resource to include OAuth type; added mcp.json to files overview
• docs/local-development.md — Added gateway environment variable injection section for local dev with deployed gateways
• README.md — Added gateway.md to documentation links

Related Issue

Documentation PR

N/A — this is the documentation PR.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Testing

How have you tested the change?

• I ran npm run test:unit and npm run test:integ
• I ran npm run typecheck
• I ran npm run lint
• If I modified src/assets/, I ran npm run test:update-snapshots and committed the updated snapshots

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the
terms of your choice.