Update to VuePress v2

[m-mohr]

VuePress v1 doesn't work well anymore with node 17+ it seems:

webpack/webpack#14532

[m-mohr]

Fixed the node version in the CI to 16.x, but this means we also can't use it locally with more recent node versions:
https://github.com/Open-EO/openeo.org/blob/master/.github/workflows/actions.yml#L14

Workaround for newer Node versions:

• Linux: export NODE_OPTIONS=--openssl-legacy-provider
• Windows: SET NODE_OPTIONS=--openssl-legacy-provide