Made Building happen async and in multiple process

[beef331]

Prior to this on my machine building the docs for nimibook took roughly around 30s it now takes around 10s. For Norm's docs it went from around 60s down to around 30s. No clue if this is an overly interesting thing, just seen the easy ability to speed it up a bit so thought I wouldd make a PR

[pietroppeter]

Thanks for this, much welcome! I was supposed to give a better look at this during the long weekend but I was not able too. I see that you are still sometimes adding commits, is this ready for review/merge or you have plans for adding/changing stuff?

[beef331]

It's probably fine now, there were issues with it prior that I realized after

[pietroppeter]

sorry about letting this sit here so long, last two/three months I had very little time for nim stuff. I have a bit of backlog to catch up now but realistically I should be able to look and merge (along with a new release of nimibook) by the end of July (last words...).

[beef331]

It's fine by me it was like less than an hour of work so it's not a big deal.

[pietroppeter]

thanks for this! had time to check it out and it does indeed work as expected. Will now merge and work on documenting last changes and adding netlify preview before releasing 0.3