Add a "When to use Flow" section to the docs

[ptodev]

One of the most common questions which the dev team gets is "how is the Agent different from the OTel Collector?".

This PR adds a new section to the docs which aims to answer this. I added this section to the very beginning of the docs:

The Flow Mode page does a great job of explaining what Flow mode is. I would like this section to be the next one, so that users can immediately see what the Agent is good at, and why they should consider it. Then, they could look into the next section - Set up Flow mode

This PR is still a draft:

• I need to add YAML and River examples in the page about OTel.
• I need to add more hyperlinks. E.g. all of the names of Flow components should be hyperlinks to the doc page of that component.
• I would like to make Venn diagrams which show how Agent relates to the Collector and the Prometheus Agent Mode.

[ptodev]

I'd like to add a Venn diagram such as this one in the beginning of this page:

[clayton-cornell]

Some first pass thoughts and suggestions.

We need to pick one name for things... it skips about between Grafana Agent... Flow.. Agent... and so on. Same with OpenTelemetry Collector... sometimes it's Collector.... the Collector... and so on.

[jpkrohling]

• You have a custom component, which you can incorporate in your own Collector distribution via the Collector Builder

[ptodev]

Is the main point the fact that you can add a custom component? Or is using the Collector Builder the advantage?

Either way, there is nothing stopping someone from forking the Agent and adding their own custom component. We just don't document that process very well, because in the long term we want to solve by developing a plugin-based system which doesn't require a new distribution/fork when a custom component is needed.

[jpkrohling]

The use case is having a custom component, having a supported (and documented?) way to do that is what determines the tooling.

[jpkrohling]

Our tests have shown that this leads to considerable performance gains.

@sh0rez , is there anything public we can link to?

[jpkrohling]

Is this the OSS documentation? One extra advantage of Flow is that Grafana Labs offers support for it.

[ptodev]

This is for OSS, but sometimes the Cloud docs import OSS docs and I'm not sure if this one will end up being included.
I wanted to write this in a more general way without mentioning support commitments, since I'm not sure if we can explain support commitments in simple terms for all users and clients who may be reading the article.

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[ptodev]

We are not quite sure where to position this in the docs, so I will close the PR. I hope I can open a similar PR in a few weeks once we have more consensus over how to convey such information.