Revise Fleet overview and deployment details

[akuthiala]

Updated description of Fleet to highlight its capabilities and deployment options.

Related issue: Resolves #

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.

• Input data is properly validated, SELECT * is avoided, SQL injection is prevented (using placeholders for values in statements)

• If paths of existing endpoints are modified without backwards compatibility, checked the frontend/CLI for any necessary changes

Testing

• Added/updated automated tests

• Where appropriate, automated tests simulate multiple hosts and test for host isolation (updates to one hosts's records do not affect another)

• QA'd all new/changed functionality manually

For unreleased bug fixes in a release candidate, one of:

• Confirmed that the fix is not expected to adversely impact load test results
• Alerted the release DRI if additional load testing is needed

Database migrations

• Checked schema for all modified table for columns that will auto-update timestamps during migration.
• Confirmed that updating the timestamps is acceptable, and will not cause unwanted side effects.
• Ensured the correct collation is explicitly set for character columns (COLLATE utf8mb4_unicode_ci).

New Fleet configuration settings

• Setting(s) is/are explicitly excluded from GitOps

If you didn't check the box above, follow this checklist for GitOps-enabled settings:

• Verified that the setting is exported via fleetctl generate-gitops
• Verified the setting is documented in a separate PR to the GitOps documentation
• Verified that the setting is cleared on the server if it is not supplied in a YAML file (or that it is documented as being optional)
• Verified that any relevant UI is disabled when GitOps mode is enabled

fleetd/orbit/Fleet Desktop

• Verified compatibility with the latest released version of Fleet (see Must rule)
• If the change applies to only one platform, confirmed that runtime.GOOS is used as needed to isolate changes
• Verified that fleetd runs on macOS, Linux and Windows
• Verified auto-update works from the released version of component to the new version (see tools/tuf/test)

[rachaelshaw]

This is the same language used for press releases, but I think we should tailor the language to this audience. "Why Fleet" should remain as approachable as possible, because this is the starting point to the documentation and we don't want to make it look more difficult.

I see the issue this PR addresses is about updating "Why Fleet" to be in line with MDM messaging. What is the smallest change we can make to this first section that achieves that?

[akuthiala]

Good for iteration 1