Document the state for the TreeList (#153)

* feat(treelist): state inititla

* chore(treelist): state docs

* chore(treelist): state article docs

* chore(treelist): publish article

* chore(treelist): fix slug

* chore(treelist): add new example to state article

* chore(treelist): add example on how to load / save state to LocalStorage

* chore(treelist): wording

* chore(treelist): fix ToC

* chore(treelist): link hierarchical data article and clarify the note

* chore(treelist): reorder inject directives

* chore(treelist): fix curly brackets indentation

* chore(treelist): fix typo in Set option through state example

* chore(treelist): add filtering from code section in the filtering article

* chore(treelist): add sorting from code section in sorting article

* chore(treelist): improve expanded items example and add tip in onstateinit section

* chore(treelist): add comment to expanded items

* chore(treelist): add column example

* chore(treelist): add relevant comments to column example

Author: svdimitr