Make docs chapter on default units and physical constants

[Eben60]

Adding documentation page on default units and physical constants, as requested in #623, and a script for creation/updating such a page. The script is to be run on the local computer computer on each update of Unitful - it will be called from docs/make.jl

Here is the new chapter as HTML rendered page.

(The script code is written without a deep understanding of the Unitful type system and is probably not exactly beautiful. However it is not intended for the end user anyway, and the result of the script is easily controlled.)

[Eben60]

bump

[Eben60]

@sostock thank you a lot for the review. I have added a couple of comments, but actually I have little experience in github collaboration, and I am afraid if I start now trying to fix each point, I have every chance to screw it here and there and it would be just more work for you. Could you probably just change everything as you consider it proper?

[sostock]

@Eben60 Yes, I can apply these changes myself. No problem!