Documentation: decl_module! macro (#2147)

* Add: initial draft of decl_module!, before fact-check

* Add: edits after content review by Guillaume

* style

* add link to event doc

* Changed `nocompile` to `rust,ignore` for docs

* Update srml/support/src/dispatch.rs

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

* Update dispatch.rs

* Changed examples layout of decl_module macro docs

* style and links

* decl_module doc tests passing

* All doc tests passing for decl_module

* Minor cleanup, comment out doc test imports

* some cleanup

* Update srml/support/src/dispatch.rs

Co-Authored-By: DemiMarie-parity <[email protected]>

* Bump `impl_version` for CI

* Fix indentation

* Update description of Multiple Module Instances

* function definition updates

* restructure example and spelling fixes

* update after review

* `ensure_root` is not a macro

* remove ! from ensure_root in text

* public vs private