AI generated docs: add source-file references to context-level accord…

[jbott-tbird]

…ions

Surface the implementing source files for each external system in the Level 1 context view via a new subtle .srcref content block.

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• This contribution does not include any changes created or assisted by AI.
• This contribution includes changes assisted by AI.
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• I have read and affirm that my contribution adheres to Mozilla’s Community Participation Guidelines
• This contribution adheres to the Thunderbird iOS Contribution Guidelines.
• This contribution does not contain merge commits, and is rebased on the latest from the iOS codebase.
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[toddheasley]

LGTM