[Docs] Add geo threshold and containment docs

[kindsun]

Adds docs for the new Geo containment and Geo threshold alert types. This PR focuses primarily on field descriptions and instructions for alert creation.

We may wish to do a follow-on PR that fleshes out an example, but since that's currently a fairly involved process which is not currently supported by sample data, this should be part of a separate effort.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)

[YulNaumenko]

LGTM

[gchaps]

For this doc to show up in the nav, add this line:

include::geo-alert-types.asciidoc[]

to the user/alerting/index.asciidoc

[kindsun]

Great comments/suggestions @nreese @gchaps! Ready for another pass whenever

[gchaps]

The doc looks good. It still doesn't show up in the nav, though (unless I am looking in the wrong place). Add this line:

include::geo-alert-types.asciidoc[]

to the user/alerting/index.asciidoc

[kindsun]

The doc looks good. It still doesn't show up in the nav, though (unless I am looking in the wrong place). Add this line:

include::geo-alert-types.asciidoc[]

to the user/alerting/index.asciidoc

I'm so sorry, I thought I had. Just updated it! 🙇

[gchaps]

LGTM

[nreese]

Is this rename really necessary?

As a user, the word "standard" does not provide a lot of meaning and obscures the real contents "alert types". I would recommend leaving this title as is. I think its ok having a title "alert types" and "geo alert types"

[kindsun]

I'll have to think about this one. I'm open to changing the wording, but it seems strange to me to have a top-level category called "Alert types" and then have another called "Geo alert types". I chose "standard" since these alert types are included at the basic license level and aren't hidden behind a feature flag.

[nreese]

LGTM, thanks for creating alerting docs