Allow disabling plugins for faster preview

[saivann]

https://github.com/bitcoin/bitcoin.org/tree/partialbuild#partial-build-for-faster-preview

This can allow contributors to significantly reduce time spent on waiting for building the website with the increasing features and languages supported.

I'll do a little more testing but so far these changes seem pretty straightforward and successfully reduce build time from ~4 minutes to ~17 seconds.

[harding]

@saivann this is super-duper-awesome! The long compile has been a huge drag. Massive thanks for this!

I've tested this and I get identical diff output[*] in the _site directory from bitcoin/master HEAD and this branch, so there are no unwanted changes. I also tested it with several different disabled combinations, and I got exactly what I expected each time. LGTM.

[*] Exception: the README.md is different, as expected.

[saivann]

@harding You should probably blame me for waiting for so long instead I guess, given how simple these changes turned out to be :) Thanks for your careful testing!

In the absence of critical feedback, this pull request will be merged on November 9th.