Update to reflect collections and lazy removal

[HCastano]

This updates the docs in order to reflect the fact that the collections and lazy
modules were removed in use-ink/ink#1111.

I've added a short page on how to initialize the Mapping, although this could be
fleshed out a bit more in the future to match the old HashMap docs (which are still
around, just hidden from the sidebar).

There's also some mentions of StorageVec in the dynamic storage allocators docs. I
think we'll be removing this in the future, so I left those pages untouched. Same goes
for Memory.

I've also fixed some of the line lengths in places, so that blows up the diff a little.

Closes #42, use-ink/ink#1128.

[HCastano]

So the CI is failing because https://paritytech.github.io/ink-docs/datastructures/mapping doesn't exist yet (was introduced in this PR)