Improve documentation for SLD calculator

[krzywon]

Is your feature request related to a problem? Please describe.
The documentation for the SLD calculator is a bit thin and could benefit with more use cases that are allowed by the periodictable package.

Describe the solution you'd like
The NIST Activation and Scattering Calculator has extensive documentation that would be easy to integrate.

Additional context
A few useful cases not covered in our documentation but covered in the online calculator:

• Mass and isotopic densities in the chemical formulas (H2O@1 -or- D2O@1n)
• Mole and mass fractions (78.2H2O[16] + 21.8H2O[18] -or- 50%wt Co // Ti)
• Biomacromolecules ("code:sequence" where code can be 'aa', 'dna', or 'rna' e.g. aa:RELEELNVPGEIVESLSSSEESITRINKKIEKFQSEEQQQTEDELQDKIHPFAQTQSLVYPFPGPIPNSLPQNIPPLTQTPVVVPPFLQPEVMGVSKVKEAMAPKHKEMPFPKYPVEPFTESQSLTLTDVENLHLPLPLLQSWMHQPHQPLPPTVMFPPQSVLSLSQSKVLPVPQKAVPYPQRDMPIQAFLLYQEPVLGPVRGPFPIIV)

This came from a discussion with @katieweigandt

[butlerpd]

It may be useful to point to the online periodictable documentation on readthedocs.io https://periodictable.readthedocs.io/en/latest/.

Also do these get too esoteric? and would it be useful to eventually provide a better GUI interface for some of the most common use cases such as mass fraction compositions?

This originated from a discussion with David Amelemah at the University of Sydney