Fix UG bugs found during PED

[yisiox]

• Fixes Missing hyperlink in table of content #253
• Fixes [PE-D][Tester D] Copy command PREFIX not shown to user in UG #251
• Fixes [PE-D][Tester D] Missing Help Command in UG #246
• Fixes [PE-D][Tester A] Existing asset being linked vs a new asset being linked #237
• Fixes [PE-D][Tester A] Database of Asset is unclear #236
• Fixes [PE-D][Tester C] Paste Keyboard on UG only matches the Windows shortcut, should include Mac Command + V #216
• Fixes Grammatical error in Features section of UG #213
• Fixes [PE-D][Tester B] Commands are not Case-sensitive #212
• Fixes [PE-D][Tester C] Removing Assets via "Edit 1 a\" does not match the behavior in the UG #207
• Fixes [PE-D][Tester E] Inconsistency in asset prefix #205
• Fixes [PE-D][Tester A] Inconsistency in format and example in "Adding a Contact: add" in UG #191
• Fixes [PE-D][Tester E] Grammatical errors in 'Using this Guide' section #187
• Fixes Missing hyperlink in table of content #253
• Fixes [PE-D][Tester E] User guide valid test case is invalid in actual JAR file #199
• Fixes [PE-D][Tester D] Inconsistent documentation of find command #217
• Fixes [PE-D][Tester C] Inconsistent Formatting of Command #189
• Fixes [PE-D][Tester B] Inconsistent Prefix Parameter in UG #190
• Fixes [PE-D][Tester B] Inconsistent UG description for Phone Num Parameter #197
• Fixes [PE-D][Tester A] Clarify the term "Modifying" #239
• Fixes [PE-D][Tester A] Lack of examples for undo command #240
• Fixes [PE-D][Tester A] Lack of Examples for Redo Command #241
• Fixes [PE-D][Tester A] UG Phrasing clarity issue with regards to assets in the edit command #230
• Fixes [PE-D][Tester B] Phone Number parameter allows + to be entered anywhere. #200
• Fixes [PE-D][Tester A] Validity of Email Field #233
• Fixes [PE-D][Tester E] Weak validation on email domain #227

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 76.94%. Comparing base (22efd1f) to head (01a60a1).

Files	Patch %	Lines
src/main/java/seedu/address/MainApp.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #255   +/-   ##
=========================================
  Coverage     76.94%   76.94%           
  Complexity      515      515           
=========================================
  Files            75       75           
  Lines          1596     1596           
  Branches        143      143           
=========================================
  Hits           1228     1228           
  Misses          338      338           
  Partials         30       30           

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[aureliony]

TODO: Write user-friendly explanation for SUBSTRING

[bryanyee33]

TODO: Write user-friendly explanation for SUBSTRING

Solved by rephrasing explanation instead.

[aureliony]

TODO: replace remaining traces of "person" with "contact"

[aureliony]

LGTM

[bryanyee33]

LGTM

[fivetran-tangyetong]

LGTM