docs: add External LLM Gateways integration page - #702

Running Vale across on the following files affected by this PR:

- enterprise/integrations/external-llm-gateways.mdx

Attempting to initialize Vale linter...
No customer vale configuration detected. Using Mintlify's default configuration.
Attempting to run Vale spellcheck...
Vale spellcheck completed successfully.

enterprise/integrations/external-llm-gateways.mdx:
	- 73:50     Did you really mean 'namespace'?
	- 251:25    Did you really mean 'passthrough'?
	- 476:26    Did you really mean 'plaintext'?


Found 3 suggested changes across 1 files in 438.13ms