[Request] Document Fleet's package management and setup process in detail

## Description

The Fleet team would like to create some documentation about Fleet's setup process in which we bootstrap various Fleet dependencies and assets like component templates, ingest pipelines, output settings, etc.

## Collaboration
- The docs team will define with product team the structure and location, and the product team will provide the initial content

I'll provide an initial outline below that should hopefully give us a good start on content here, but we should determine the specifics about where/how to present this documentation in the comments.

**Contact Person:** @kpollich

## Suggested Target Release

N/A

## Stakeholders

- @joshdover
- @mostlyjason

Also cc @nchaulet for input and guidance.

_Content outline provided below_

## Fleet setup

Fleet runs a series of setup steps when Kibana starts up to bootstrap various dependencies and apply any [preconfiguration settings](https://www.elastic.co/guide/en/kibana/current/fleet-settings-kb.html#_preconfiguration_settings_for_advanced_use_cases) specified in `kibana.yml`. These steps are listed in the order of their execution below.

Note: we have a separate issue to improve our preconfig docs here https://github.com/elastic/docs-content/issues/7788

### Default settings

Fleet configures a few defaults for [settings](https://www.elastic.co/guide/en/fleet/current/fleet-settings.html) if they are not provided via preconfiguration or in the case of a first-time setup where settings have not been set in the UI.

- In cloud, Fleet resolves a Fleet Server host URL automatically and sets it as a default
- Fleet sets a default [output](https://www.elastic.co/guide/en/fleet/current/elasticsearch-output.html) pointed at Elasticsearch

### Elasticsearch assets

Fleet creates the following Elasticsearch assets:

- A default [component template](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-component-template.html) named `.fleet-component-template-{version}` that contains common index settings and mappings for data sent via Elastic Agent
- An [ingest pipeline](https://www.elastic.co/guide/en/elasticsearch/reference/master/ingest.html) named `.fleet-final-pipeline-1` that applies some final processing to data sent via Elastic Agent

### Integration installation and updates

- Install any integrations specified in preconfiguration
- Determine any installed integrations that qualify for automatic updates and install their latest versions

#### Bundled integrations

In 8.1, Kibana will begin shipping with "bundled" integrations to support offline upgrades for select Elastic product integrations like [APM](https://www.elastic.co/guide/en/observability/current/apm.html) and [Synthetics](https://www.elastic.co/guide/en/observability/current/monitor-uptime-synthetics.html).

When Kibana is upgraded and a new version of one of these bundled integrations is detected in the Kibana source, it will be installed from disk, and will not require an available connection to the Elastic Package Registry. This improves support for [air-gapped environments](https://www.elastic.co/guide/en/fleet/current/air-gapped.html) and ensures stability when upgrading and booting Kibana.

As of 8.1, these bundled integrations are:

- APM
- Elastic Agent
- Endpoint
- Fleet Server
- Synthetics

### Package policy upgrades

Fleet will detect any integrations with the "Keep Policies up to Date" [setting](https://www.elastic.co/guide/en/fleet/master/upgrade-integration.html#upgrade-integration-policies-automatically) enabled and apply any policy upgrades in the case where these integrations have been updated.

### Default enrollment token

Fleet will create an [enrollment token](https://www.elastic.co/guide/en/fleet/current/fleet-enrollment-tokens.html) for any preconfigured agent policies.