Update developer guide

[derekjxtan]

Sections updated:

1. Design (Architecture) closes #277
2. Design (Model Component) closes #280
3. Design (Storage Component) closes #281
4. NFR
5. Appendix: Instructions for manual testing (Saving data)

[codecov[bot]]

Codecov Report

Merging #290 (84fe77a) into master (873b3b3) will decrease coverage by 0.12%. Report is 22 commits behind head on master. The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #290      +/-   ##
============================================
- Coverage     80.79%   80.68%   -0.12%     
  Complexity      938      938              
============================================
  Files           137      137              
  Lines          2833     2837       +4     
  Branches        275      277       +2     
============================================
  Hits           2289     2289              
- Misses          511      513       +2     
- Partials         33       35       +2     

see 1 file with indirect coverage changes

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