Add migration guide from ink! 4.x to 5.0 (#301)

* Add migration guide from ink! 4.x to 5.0

* Add Krest to compatible production chains

* Add screenshot of `cargo contract storage`

* Add steps to upgrade

* Add breaking decode behavior change

* Add note for `StorageVec`

* Add note on support for multiple chain extensions

* Add note on stabilized `call_runtime`

* Add note on chain extension syntax change

* Move `StorageVec` section up

* Add note on new fallible methods

* Move `StorageVec` and fallible methods to important section

* Improve docs on fallible methods

* Improve `StorageVec` text

* Mention `non_fallible_api` lint

* Update list of fallible `StorageVec` methods

* Update text on recommendations

* Update section on chain extensions

* Mention contract verification

* Add note on fallible methods

* Apply suggestions from code review

Co-authored-by: Gherman <[email protected]>

* Explain reason for data migration better

* Move custom signature topics to Events 2.0

* Added E2E builder API changes

* Link to more infos about safe math in Rust

* Improve text on safe math

* Clarify text

* Improve text

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Co-authored-by: Gherman <[email protected]>