[DOCS] Documentation for user

[zhao-cy]

• how to install libhdf5-dev package on a Mac computer: $ brew install hdf5 <== update this in Readme.Rmd (NOT .md on frontpage!!!)
• docs of functions
  • add explanations of stat outputs: link to broom package's explanations (lm, gam respectively)
  • add a table explaning var.*, full.outputs, correct.p.values.*
• demos
  • main page !!
    • display on main page of github
    • display on main page of github.io/modelarray
  • main scope of ModelArray
    • add example of p.value corrections for ModelArray.gam() - as it's different from .lm()
  • major part of doc for user:
    • make this sentence clearer (per Joelle suggestion): The requirement of this .csv: There must be a column of “source_file”, providing the relative filepath of the source image file. This requirement is consistent with that in ConFixel - please see ConFixel's docs [link] for how to set up the .csv file if you haven't run ConFixel already.
    • example of smooth term with interaction term (e.g. s(age, by=oSex)
  • side topics, e.g. basic uses of R
    • how to check if a model is well-conditioned
• check pkgdown's display (github.io/modelarray):
  • fix the quick view page (e.g. in slack): "What the package does (one paragraph)."
  • fix "on this page": "#" is not displayed, but "##" does. Try lower all levels by one.
• pkgdown:
  • Reference section:
    • not to list all functions, but select the key functions, and organize them. Ref: example: pkgdown function reference; source code; how to write;
    • remove () for two functions...