Fix inconsistencies in UGDG

[qreoct]

Fixed a couple of inconsistencies in UG/DG:

Formatting

• Standardize all h2s to be bolded so it looks nice on the Table of Contents
• Changed horizontal rule to --- in markdown so it's easier to type
• Standardize image size to not appear squashed
• Made the Alternative 1, Alternative 2, and Current choice consistent for DG Implementation

Clarity

• Wrote more Manual Tests in DG, cleaned up that section
• Fixed some inconsistencies in UG (e.g. rt/NEW_TAG -> rt/OLD_TAG) for remove tag parameter in Edit command)
• Clarified that flags are all optional in UG -> How to Read Commands -> Flags. This is because there was some confusion in Command Summary in UG where it seems that flags are compulsory.
• Clarified StudyTrackerParser#isInvokingAlias() function.

Preview

[codecov-commenter]

Codecov Report

Merging #246 (013277b) into master (859693a) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #246   +/-   ##
=========================================
  Coverage     68.19%   68.19%           
  Complexity      745      745           
=========================================
  Files            94       94           
  Lines          2581     2581           
  Branches        346      346           
=========================================
  Hits           1760     1760           
  Misses          763      763           
  Partials         58       58           

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