Documentation for assets module

[ltfschoen]

Relates to polkadot-developers/substrate-developer-hub.github.io#22

[ltfschoen]

@bkchr I've pushed commits to address your review comments.

[kianenigma]

@ltfschoen based on the new discussion progress the file should be changed to .md

[shawntabrizi]

@ltfschoen Also the template has changed a bit, and I think this does not quite reflect that

[ltfschoen]

@kianenigma @shawntabrizi @xlc @bkchr I've pushed a commit to address all comments.

Run cargo doc --package srml-assets --open to view the docs

[ltfschoen]

@joepetrowski I've addressed your comments and the indentation. This comment is outstanding: #1945 (comment)

[joepetrowski]

Changes LGTM, should get resolution on the "goals" section and approval from code author before merging.

[joepetrowski]

Updated links, style, shortened a few things to match other modules. Should consider removing the ### Events section entirely as no other SRML module includes it.

[ltfschoen]

@bkchr who would be most appropriate to get resolution on the "goals" section, and who would be the most appropriate code author to provide approval, as mentioned here #1945 (comment)

[bkchr]

@ltfschoen this documentation is not following the standard like the others. Then there are code example that will not be tested by rustdoc.

[gavofyork]

This is kind of pointless test code as it doesn't demonstrate how to use the three transaction functions. But I guess there's a bit more detail than before. I removed the obviously pointless bits of code (getters that didn't actually get) and will merge.