Update documentation

[ZhangWanlin98]

• Added Commands write up
• Added Natural Date & Time section under command format
• Updated FAQ with a link to Date & Time format
• Added glossary
• Updated command diagrams in user guide
• Updated PPP

[codecov-io]

Codecov Report

Merging #198 (6a657c0) into master (6015043) will increase coverage by 6.19%.
The diff coverage is 36.36%.

@@             Coverage Diff              @@
##             master     #198      +/-   ##
============================================
+ Coverage     74.78%   80.98%   +6.19%     
- Complexity      943     1037      +94     
============================================
  Files           125      125              
  Lines          2951     2982      +31     
  Branches        376      382       +6     
============================================
+ Hits           2207     2415     +208     
+ Misses          640      468     -172     
+ Partials        104       99       -5     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...a/seedu/address/logic/parser/ArgumentMultimap.java	100.00% <ø> (+14.28%)	9.00 <0.00> (+2.00)
...rc/main/java/seedu/address/model/ModelManager.java	98.27% <ø> (+9.48%)	57.00 <0.00> (+5.00)
src/main/java/seedu/address/model/UserPrefs.java	98.18% <ø> (+14.25%)	23.00 <0.00> (+5.00)
...ain/java/seedu/address/model/appointment/Time.java	95.23% <ø> (-0.42%)	14.00 <0.00> (-3.00)
...seedu/address/model/interval/TimeIntervalList.java	86.20% <0.00%> (-6.39%)	12.00 <0.00> (ø)
...rc/main/java/seedu/address/model/patient/Nric.java	66.66% <ø> (ø)	7.00 <0.00> (ø)
...address/storage/archive/CsvAppointmentArchive.java	92.85% <ø> (+8.92%)	18.00 <0.00> (+4.00)
...rc/main/java/seedu/address/ui/AppointmentCard.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...in/java/seedu/address/ui/AppointmentListPanel.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
... and 38 more

---

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[YangYue128-helen]

Other than a minor phrasing issue, LGTM!
We can see whether your images look ok after merging.

[JinHao-L]

Other than some minor details, the rest LGTM 👍