Update UG and DG with Delete and Stats features, Fix some PED Documentation Bugs

[butteredyakiimo]

• Make some PED bug fixes (see pic in tele)
• Standardise notations used in UG
• Add some planned enhancements
• Edit use cases and user stories
• Modify manual testing in DG
• Add in implementation details for Delete and Stats

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fe0efc1) 61.69% compared to head (31d2129) 61.69%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #139   +/-   ##
=========================================
  Coverage     61.69%   61.69%           
  Complexity      703      703           
=========================================
  Files           104      104           
  Lines          2872     2872           
  Branches        318      318           
=========================================
  Hits           1772     1772           
  Misses         1012     1012           
  Partials         88       88           

Files	Coverage Δ
...java/seedu/address/commons/core/index/Indices.java	94.00% <ø> (ø)

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[nananakx-x]

Did not really read the UG changes in detail, but will definitely do so when I am adding on later! We should merge this and later check if we still have formatting errors and I can change them later :)

[nananakx-x]

lgtm