Add documentation and tests for dollar sign escaping in config

[robert113289]

This is a documentation PR that adds a unit test. No production code changes.

This PR documents the $$ escape syntax for including literal dollar signs in router YAML configuration values. This syntax is supported by the underlying shellexpand crate but was previously undocumented, causing user confusion in apollographql/rover#2737 .

• Adds new "Escaping special characters" section to the YAML configuration reference
• Example: some$$api$$key → some$api$key

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* graphos/routing/(latest)/configuration/yaml.mdx

Build ID: c6b32651f7b0048548f199e9
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URL: https://www.apollographql.com/docs/deploy-preview/c6b32651f7b0048548f199e9

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