feat(docs): optimized style and script loading

[mxschmitt]

Changes:

• Moved all external assets into local bundle
• Reduced resource size from 1.9 MB to 1.7 MB at e.g. Alerts page (I know, that's less, but we now not using external CDNs => DNS etc.)
• Now using Bootstrap 4.3.1 (latest) instead of 4.2.1 for the docs

[taion]

cc @jquense, because i don't understand the idiomatic way to do this with gatsby

[jquense]

this seems like a good change to me 👍 I don't think gatsby has an opinion here on a better way.