[EuiCollapsibleNavBeta] Add support for a non-clickable, non-collapsible group heading

[ryankeairns]

Is your feature request related to a problem? Please describe.
This surfaced in the nav design for Serverless. As the solution group will be the main and only group (other than the footer), we desire to not have it appear as collapsible.

Describe the solution you'd like
A group heading, in the EuiCollapsibleNavBeta, that is neither clickable nor collapsible. End result looking like:

Describe alternatives you've considered
We considered existing props to force it open, but accessibility concerns surfaced.

Desired timeline
This is for Serverless so mid-October (the 15th, ideally) availability in Kibana.

Additional context
The quote below is pulled from See this thread:

Can you rethink this ask? If it's truly crucial, the component used essentially can't be an accordion. Hiding the arrow is fine for visual users but there's still a ton of accessibility implications of accordions that's going to confuse screen reader users if we leave it in.

If we need a way to render a header with items that's not collapsible, that's fine, but I need to render it a different way than using an underlying EuiAccordion.

[ryankeairns]

Let's chat quick once you crack this open. There are some additional padding changes we would like to make that could be (I will eat these words) simple to make. Basically, reduce padding to the left of the link list and add some padding around this new 'static' group heading (preview below). If it is anything more than simple, then I will create a separate issue. I swear.

Update: this is just me messing around in the browser as a rough estimate

(increase top and bottom padding on the parent)

(decrease start; increase end padding on the children)

(end result)

[cee-chen]

ETA probably next week/Monday for picking this up! Feel free to throw something on my calendar if you think it needs more discussion, but I can also just go off the Figma file. Thanks for the heads up!

[cee-chen]

@ryankeairns Should the group heading title (icon + solution name) be able to be a clickable link, or will it always be static text?

[ryankeairns]

@ryankeairns Should the group heading title (icon + solution name) be able to be a clickable link, or will it always be static text?

@cee-chen Static, with a clarifying point: this top grouping is different from the groupings in the footer. The latter will need to be clickable while the top (solution) grouping would not be.

For those following along
Cee and I were discussing the long-term handling of this new component during the EUI sync. For reasons, it is becoming quite opinionated. As a result, it may be that we end up reconsidering where this lives (e.g. in Kibana as a shared component vs in EUI). Let's come back to that once we get past the October 25th target and delve further into the long-term nav design/flyout solution (which also would impact the footer).

[cee-chen]

@cee-chen Static, with a clarifying point: this top grouping is different from the groupings in the footer. The latter will need to be clickable while the top (solution) grouping would not be.

FYI, I'm making this new EuiCollapsibleNavGroup component a total separate component from the current EuiCollapsibleNavItem component (frankly, primarily because it smells like a weird one-off to me). So just another FYI I can't enforce that at the EUI level (whether or not devs use the top-level group in the body or the footer is something Kibana will need to enforce).