Make Dokka task independent on accessibility of external API

[qwwdfsad]

We need to write a small tool(?) that fetches all external package-list files for the documentation we link to and store it somewhere so that they are committed to our repo. Dokka configuration, in turn, shall point to those local file, so that build is independent from availability of 3rd party documentation servers.

[elizarov]

We still need to write it. Currently, when building documentation with gradlew dokka (which is also required for knit and for making a release), external dependencies package lists are fetched from internet and when any of those site is not responsive you cannot build docs, thus cannot make release.