Move Agent policy settings docs to correct location

[kilfoyle]

Some settings were missing or documented in the wrong location in the docs. This PR:

• Moves the "Host name format" setting to the agent policy settings page, here.
• Adds the missing "Agent binary download" setting to the to the agent policy settings page, here.
• Adds the missing "Proxy" setting to the Fleet Server settings page, here.

@juliaElastic, @criamico The content isn't really new but rather just moved around. Would you mind taking a look just to make sure the docs are okay?

Closes: #1091

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[kilfoyle]

@elasticmachine run docs-build

[mergify]

This pull request is now in conflict. Could you fix it @kilfoyle? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch <remote-repo>
git checkout -b 1091/move-settings <remote-repo>/1091/move-settings
git merge <remote-repo>/main
git push <remote-repo> 1091/move-settings

[kilfoyle]

Opened a security docs PR to fix the failing docs CI check.

[kilfoyle]

@elasticmachine run docs-build

[karenzone]

LGTM

[criamico]

LGTM 🚢