doc: improved documentation for near-sdk and near-sdk-macros crates

[PolyProgrammist]

Resolves: #1294

• Improved documentation for near-sdk crate (adding more link, actualize documentation, extending examples)
• Added documentation for near-sdk-macros crate (near, near_bindgen, etc)
• Actualize Readme.md file

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.26%. Comparing base (0027201) to head (3115a09).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1262      +/-   ##
==========================================
+ Coverage   80.24%   80.26%   +0.01%     
==========================================
  Files         104      104              
  Lines       14819    14819              
==========================================
+ Hits        11892    11894       +2     
+ Misses       2927     2925       -2     

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[dj8yfo]

@PolyProgrammist suggest adding more links, mentioned in #1265 and marked up in review of #1259,
in addition to ones, added in scope of current pr, after fixing conflicts, if you plan to finish current pr

[PolyProgrammist]

Which links do you mean? @dj8yfo

[PolyProgrammist]

Added half of the comments. Left the other for the future

[denbite]

Great job on this, I've dropped a few comments with some thoughts on how it could be even better
it's mainly focused on adding more examples

[PolyProgrammist]

@dj8yfo @denbite thanks for the review. Please check again

[akorchyn]

Overall good job. However, I added a couple nits and one major point to fix (use the near macro in root macro)

[akorchyn]

@race-of-sloths score 13

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Current status: executed

Reviewer	Score
@akorchyn	13

Your contribution is much appreciated with a final score of 13!
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