Update ug 1.4

[nicleejy]

Update the UG from the features section onwards.

Changes made:

1. Added demo diagrams to show how the commands work
2. Added key definition of terms like Category, Expense and Recurring Expense
3. Added warnings and info boxes to the relevant commands
4. Update with expense statistics feature
5. Added FAQ section
6. Fixed broken hyperlinks

Closes #205
Closes #199
Closes #197
Closes #196
Closes #193 (Different behavior is explained in UG)
Closes #164
Closes #184 (CLEAR behavior is explained in UG)

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2c77d11) 47.49% compared to head (a81cc43) 47.49%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #213   +/-   ##
=========================================
  Coverage     47.49%   47.49%           
  Complexity      491      491           
=========================================
  Files           109      109           
  Lines          2472     2472           
  Branches        262      262           
=========================================
  Hits           1174     1174           
  Misses         1193     1193           
  Partials        105      105           

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