Add support for 'extra' mkdocs config option

[stephffuller]

Using variables with the extra config option for mkdocs results in errors when trying to create pdfs with mkdocs-exporter. Please add support for the 'extra' config option.

[adrienbrignon]

Hello @stephffuller,

Thank you for raising this issue.

Could you please share the specific errors you're encountering? Additionally, if possible, a small reproduction of the problem would be very helpful.

I've attempted to replicate the issue on my end but haven't been successful. From my understanding, variables typically don't render without using a plugin like macros. However, everything seems to work fine when this plugin is enabled.

Looking forward to your response.

Thank you!

[stephffuller]

@adrienbrignon I switched computers since I began troubleshooting these issues. I have to set the entire thing up again to try this out. We just finished a sprint so I have some breathing room now. I'll get back to you ASAP on this. Haven't forgotten about contributing to the docs either :)