iOS Development guide: update readme

[nancywu1]

• add links in beginning to link to section
• add note about building all targets

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.81%. Comparing base (a7f9b32) to head (e6cc765).
Report is 3 commits behind head on bazel-6.

Additional details and impacted files

@@             Coverage Diff             @@
##           bazel-6     #386      +/-   ##
===========================================
+ Coverage    91.79%   91.81%   +0.02%     
===========================================
  Files          339      339              
  Lines        27025    27009      -16     
  Branches      1954     1954              
===========================================
- Hits         24808    24799       -9     
+ Misses        2204     2197       -7     
  Partials        13       13              

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[nancywu1]

/canary