Documentation for example module (#1947)

* WIP: SRML Example Module README

* add newlines

* review-fix: Change const to let. Explain generic usage more

* refactor: Remove example steps 2 and 3. User can refer to other examples to figure it out

* fix: Update to incorporate approved approach of staking module docs in PR #1951

* fix: Move into expandable Details arrow and fix syntax so appears correctly in rust docs

* fix: Fix linting

* docs: Add Public Dispatchable functions

* fix: Rearrange to use Simple Code Snippet and Examples from SRML

* fix: Remove duplicate Dispatchable Functions section

* fix: Remove Implementation Details as requested by Gav