docs: add explanation page for modes of operation

[Thanhphan1147]

Applicable spec:

Overview

Rationale

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated
• The PR is tagged with appropriate label (urgent, trivial, senior-review-required, documentation)
• The docs/changelog.md is updated with user-relevant changes.

[erinecon]

Thanks so much for adding this explanation document about the different modes of operation! I think this page will provide a lot of clarity for our users :D

[erinecon]

Suggested change

	There are three modes, organised by the underlying route-provider interface and
	There are three modes, organized by the underlying route-provider interface and

nit, US English

[erinecon]

I'm wondering whether this should be a Mermaid diagram -- I think the connections might be clearer (for example, it's not immediately obvious to me what is the difference between <-- and --).

Did you have any motivating factors for not using a Mermaid diagram here?

[erinecon]

Same question about whether a Mermaid diagram should be used here

[erinecon]

Same question about whether a Mermaid diagram should be used here

[erinecon]

Suggested change

	address and port through charm configuration:
	address and port through charm configurations:

Unless a single configuration sets the backend address and port, I think this should be plural

[erinecon]

Nit: I think it might be a little misleading to say that the how-to guide shows a worked example since the guide has placeholders for e.g. the backend IP.

So I think we should rephrase this sentence, maybe something like:

Suggested change

	See {ref}`how_to_haproxy_integrate_non_charm_workload` for a worked example.
	See {ref}`how_to_haproxy_integrate_non_charm_workload` for instructions on
	using `ingress-configurator` in HAProxy integrator mode.

I appreciate any other suggestions or thoughts here!

[erinecon]

Same nit about being a bit misleading with "worked example". Maybe we can update to:

Suggested change

	`ingress` interface alone. See {ref}`how_to_add_haproxy_features_to_ingress_requirer`
	for a worked example.
	`ingress` interface alone. See {ref}`how_to_add_haproxy_features_to_ingress_requirer`
	for instructions on using `ingress-configurator` in HAProxy adapter mode.

[erinecon]

Suggested change

	{ref}`how_to_gateway_api_add_features_to_ingress_requirer` for a charmed backend.
	{ref}`how_to_add_gateway_api_features_to_ingress_requirer` for a charmed backend.

Okay this is the issue with the RTD build (not the other label, that one is fine)

[erinecon]

Suggested change

	See {ref}`how_to_gateway_api_integrate_non_charm_workload` for a worked example
	See {ref}`how_to_gateway_api_integrate_non_charm_workload` for instructions

Nit, to remove "worked example" which I feel is a bit misleading