Add docs chapter on default units and physical constants

[Eben60]

Adding (only) documentation page on default units and physical constants, as requested in #623 - a temporary alternative to generation of this documentation as suggested in #704.

The PR #704 is somewhat stalled: Having the script to do exactly what we want might take time.

I have now manually edited the (initially generated) doc page according to @sostock remarks (#704 (comment), #704 (comment), #704 (comment)).

Could we probably merge this docs page as a provisional solution?

[Eben60]

Any chance?

I created this because of the frustration due to lack of units documentation: the only way to find out being studying the source code. As Unitful belongs to the most used packages in the Julia ecosystem, one can safely assume I was not alone with my frustration.

I understand automatic generation of the docs as in my PR #704 could in theory have some unexpected implications, must be thoroughly checked etc. But what could speak against merging just the docs page according to the current PR?

☹️