Update Userguide, Ui.png and Sample Data

[yejiadong]

• Updated Userguide with current find-training command implementation
• Edited bugs in Userguide
• Updated Ui.png to reflect latest GUI
• Updated Sample Data to also add trainings and attendances for better testing experience

[codecov-io]

Codecov Report

Merging #154 into master will decrease coverage by 0.89%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #154      +/-   ##
============================================
- Coverage     69.12%   68.22%   -0.90%     
  Complexity      789      789              
============================================
  Files           122      122              
  Lines          2675     2710      +35     
  Branches        336      341       +5     
============================================
  Hits           1849     1849              
- Misses          702      737      +35     
  Partials        124      124              

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/seedu/canoe/model/student/Student.java	72.47% <0.00%> (-4.32%)	53.00 <0.00> (ø)
...in/java/seedu/canoe/model/util/SampleDataUtil.java	9.25% <0.00%> (-7.99%)	2.00 <0.00> (ø)

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[sudogene]

LGTM