Document that `markdown.shikiConfig.theme` can be a relative path

[delucis]

It came up in a support thread recently that Shiki themes can be loaded by providing a relative path: https://discord.com/channels/830184174198718474/1026227648055222363/1026235701777412157

Would be great to add a small example to the Markdown guide as it’s not obvious what such a path should look like, but this is often nicer than passing lots of config options.

[nujovich]

Can I be assigned to this? I'd love to help

[kevinzunigacuellar]

Can I be assigned to this? I'd love to help

You got it!