Update Developer Guide and UML Diagrams

[cleowenxuan]

1. Included a proposed feature: Deleting by name or index under the Implementations section inside the Developer Guide
2. Modified the DeleteSequenceDiagram
3. Created a UML Diagram called DeleteActivityDiagram that is used inside the Developer Guide

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.54%. Comparing base (9e826f4) to head (8983b04).

Additional details and impacted files

@@              Coverage Diff              @@
##             master      #46       +/-   ##
=============================================
+ Coverage          0   69.54%   +69.54%     
- Complexity        0      471      +471     
=============================================
  Files             0       81       +81     
  Lines             0     1609     +1609     
  Branches          0      165      +165     
=============================================
+ Hits              0     1119     +1119     
- Misses            0      447      +447     
- Partials          0       43       +43     

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[peienlim]

LGTM