Documentation for Contract Module (#2045)

* Add: documentation for srml_contract, per @sergeis comments. TODO: make concise and add usage example

* Fix: minor spelling and grammar

* Add: Revisions to the doc clarification from @Sergei on trx revert behavior

* Edit: added spacing btw function list for better readability

* Remove: usage create contract example todo later

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Edit: explained max gas limit revert behavior in greater detail

* Edit: incorporated first round of PRs, mostly syntax/styling

* Edit: incorporated Sergeis mental model, terms, and added usage examples

* Fix: staking module is no longer closely related to contract. Balances now manages the reaping of accounts

* Minor cleanup of links/text that are not ready to be shown

* Update srml/contract/src/lib.rs

made more concise

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Minor edits from PR

* Explained contract creation in more detail, fixed PR suggestions

* Linted according to contri guidelines

* Created a notable scenarios section, unlisted public functions in lieu of a direct link to the module

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/tests.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* Update srml/contract/src/lib.rs

Co-Authored-By: nczhu <[email protected]>

* minor commit after pr feedback

* s/codehash/code_hash for consistency