Documentation fixes and additions - installation related

[mliu49]

Motivation or Problem

To make the installation instructions easier to read and reference.

Description of Changes

• Fix spacing/indentation issues
• Convert bullet points to numbering to allow easier referencing
• Update dependencies list in documentation
• Move installation FAQ to main FAQ page
• Add more FAQs
• Make Arkane.py executable
  (Reviewer note: since this is very unrelated, let me know if you would prefer this to be in a separate PR)

Testing

Make the html documentation and look at the modified pages.

[codecov]

Codecov Report

Merging #1567 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1567      +/-   ##
==========================================
+ Coverage   41.85%   41.87%   +0.01%     
==========================================
  Files         165      165              
  Lines       28004    28004              
  Branches     5713     5713              
==========================================
+ Hits        11721    11726       +5     
+ Misses      15494    15490       -4     
+ Partials      789      788       -1

Impacted Files	Coverage Δ
rmgpy/data/kinetics/family.py	58.56% <0%> (+0.29%)	⬆️

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[amarkpayne]

Everything is showing up on my end as you intended, and the numbering system is far superior to our old bullet point system. I am also fine with the commit to make Arkane.py executable--I think it is closely related enough as to be included in this PR. Finally, I also checked that everything in the FAQ section got moved over properly. I'll go ahead and merge this--thanks for the great PR!!