docs(store): add docs about state operators

[arturovt]

PR Checklist

• The commit message follows our guidelines: https://github.com/ngxs/store/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

#786

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[eranshmil]

You also need to add a link to this new page. :)

Edit: And maybe add links to Mark's medium post and the main issue.

[arturovt]

@markwhitfeld @deebloo gentlemen

[arturovt]

@markwhitfeld @eranshmil @splincode updated

[eranshmil]

Just add the example back as discussed in the comments.

Edit: And add the link in the docs menu.

[arturovt]

@eranshmil Sure, I've just missunderstood, should I replace patch with:

const state = ctx.getState();

ctx.setState(state => ({
  ...state,
  monkeys: []
}));

[markwhitfeld]

const state = ctx.getState();
ctx.setState({
  ...state,
  monkeys: []
});

[markwhitfeld]

Please also add a link to this page as the first item under the advanced menu section. You can label it "State Operators".

[arturovt]

@markwhitfeld 🚀

[markwhitfeld]

Sorry for the delay. It's been a busy few days.