Revert "docs: add gateways and proxies section to README"

[nirinchev]

Reverts mongodb-js/mongodb-mcp-server#438

[Copilot]

Pull Request Overview

This PR reverts a previous commit that added a comprehensive "Gateways and Proxies" section to the README documentation. The revert removes detailed documentation about authentication gateways, reverse proxies, and specific third-party solutions like Pomerium, while restoring a simpler "Proxy Support" section that focuses only on outbound proxy configuration.

• Removes extensive gateway and proxy documentation including security best practices and Pomerium integration details
• Restores the original simpler "Proxy Support" section with basic outbound proxy environment variable configuration
• Updates table of contents to reflect the change from "Gateways and Proxies" back to "Proxy Support"

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

The word 'behaviour' should be spelled 'behavior' to maintain consistency with American English spelling used elsewhere in the documentation.

Suggested change

	to third-party services like OID Providers. The behaviour is the same as what
	to third-party services like OID Providers. The behavior is the same as what

[coveralls]

Pull Request Test Coverage Report for Build 17078670712

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 82.143%

---

Totals
Change from base Build 17076257943:	0.1%
Covered Lines:	4242
Relevant Lines:	5139

---

💛 - Coveralls