chore: update contributing guide for push command with proxy

[kanoru3101]

What/Why/How?

Added a section on how to run the push command through a proxy server.

Reference

Internal

Testing

Screenshots (optional)

Check yourself

• Code changed? - Tested with Redoc/Realm/Reunite (internal)
• All new/updated code is covered by tests
• New package installed? - Tested in different environments (browser/node)
• Documentation update considered

Security

• The security impact of the change has been considered
• Code follows company security practices and guidelines

[changeset-bot]

⚠️ No Changeset found

Latest commit: d72b0cc

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[github-actions]

Command	Mean [s]	Min [s]	Max [s]	Relative
redocly lint resources/rebilly.yaml	1.533 ± 0.017	1.507	1.560	1.02 ± 0.03
redocly-next lint resources/rebilly.yaml	1.501 ± 0.033	1.444	1.564	1.00

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	78.58% (🎯 78%)	6185 / 7870
🔵	Statements	78.01% (🎯 78%)	6390 / 8191
🔵	Functions	81.97% (🎯 81%)	1264 / 1542
🔵	Branches	70.39% (🎯 70%)	4239 / 6022

File Coverage

No changed files found.

Generated in workflow #8205 for commit d72b0cc by the Vitest Coverage Report Action

[kanoru3101]

I am closing PR as there are not many details to add, so there is not much point in describing obvious things that can be found in the push command.