Update User Guide

[sagarsureka]

Add to the introduction section
Add a "About this document section"
Add links to all the subheading to navigate the user guide better

[rolandyuwy]

A few minor suggestions, thanks!

[codecov-io]

Codecov Report

Merging #123 into master will increase coverage by 2.33%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #123      +/-   ##
============================================
+ Coverage     68.78%   71.11%   +2.33%     
- Complexity      545      585      +40     
============================================
  Files            89       89              
  Lines          1797     1835      +38     
  Branches        201      205       +4     
============================================
+ Hits           1236     1305      +69     
+ Misses          478      442      -36     
- Partials         83       88       +5     

Impacted Files	Coverage Δ	Complexity Δ
...eedu/simplykitchen/logic/commands/EditCommand.java	96.96% <0.00%> (-0.05%)	12.00% <0.00%> (ø%)
.../java/seedu/simplykitchen/model/food/Priority.java	79.16% <0.00%> (ø)	16.00% <0.00%> (+5.00%)
...main/java/seedu/simplykitchen/model/UserPrefs.java	81.57% <0.00%> (+1.57%)	12.00% <0.00%> (+2.00%)
...n/java/seedu/simplykitchen/model/ModelManager.java	99.06% <0.00%> (+2.25%)	37.00% <0.00%> (+3.00%)
.../java/seedu/simplykitchen/model/FoodInventory.java	87.87% <0.00%> (+6.06%)	15.00% <0.00%> (+1.00%)
...seedu/simplykitchen/model/food/UniqueFoodList.java	93.33% <0.00%> (+8.88%)	22.00% <0.00%> (+2.00%)
...seedu/simplykitchen/model/util/ComparatorUtil.java	70.58% <0.00%> (+29.41%)	14.00% <0.00%> (+11.00%)
...va/seedu/simplykitchen/model/food/Description.java	69.56% <0.00%> (+33.56%)	14.00% <0.00%> (+7.00%)
.../simplykitchen/logic/commands/SortDescCommand.java	100.00% <0.00%> (+100.00%)	3.00% <0.00%> (+3.00%)
...implykitchen/logic/commands/SortExpiryCommand.java	100.00% <0.00%> (+100.00%)	3.00% <0.00%> (+3.00%)
... and 1 more

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[rolandyuwy]

LGTM