Fetch code snippets from local sources/Gridsome instead of Github, for all code except external API references/proto files

[jorunfa]

Currently there is a bit of hen vs egg problem; we try to fetch code that might not yet have been pushed to GitHub yet.

But there is also a bit of consistency problem. If the code you see is not the same as the one that you see when you click the link, that's also weird. But that should only be the case when working locally.

[jorunfa]

Relevant tech/docs:

• https://gridsome.org/plugins/gridsome-remark-embed-snippet
• https://gridsome.org/docs/fetching-data/
• https://gridsome.org/plugins/@gridsome/source-filesystem