doc: #[near(contract_state)] in-depth pass

[dj8yfo]

• basic example: add impl block
• resolves doc: missing mentions in glossary of rustdoc of IterableMap, IterableSet #1283
• store: anchor for host-functions (env), used in impl
• store: add faq heading about flush+Drop
• collections: anchor for host-functions (env), used in impl
• resolves doc: #[near(contract_state)] doc should link to [store] and [collections] modules #1304
• resolves doc: #[near(contract_state)] subheading about implementation details + used host functions #1306
• abi workaround

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.06%. Comparing base (1f3c071) to head (87ea9ad).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1307      +/-   ##
==========================================
- Coverage   80.19%   80.06%   -0.14%     
==========================================
  Files         104      104              
  Lines       14856    14856              
==========================================
- Hits        11914    11894      -20     
- Misses       2942     2962      +20     

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[dj8yfo]

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