[Fleet] Update landing page for add fleet server

[criamico]

Summary

Follow up of #142894
Part of #137785

Updates the landing page for "Add fleet server":

"Add fleet Server button" opens up the flyout:

Also the "add fleet server" in settings section opens up the flyout:

Checklist

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)

[criamico]

@elasticmachine merge upstream

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[criamico]

@elasticmachine merge upstream

[nchaulet]

In the design looks like the landing page text has a max width maybe something we should keep here

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 5e2f1cd
• Storybooks Preview

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
fleet	855.9KB	858.0KB	+2.1KB

Unknown metric groups

ESLint disabled in files

id	before	after	diff
osquery	1	2	+1

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
fleet	58	64	+6
osquery	103	108	+5
securitySolution	439	445	+6
total			+19

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
fleet	66	72	+6
osquery	104	110	+6
securitySolution	516	522	+6
total			+20

History

• 💔 Build #84799 failed aa11740
• 💔 Build #84592 failed 16112ca
• 💔 Build #83602 failed 2cc2056

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @criamico

[nchaulet]

LGTM!

[juliaElastic]

Is this change in scope? #144242 (comment)

[criamico]

Is this change in scope? #144242 (comment)

@juliaElastic we discussed with @nchaulet and we decided to go ahead with what we already have and do any further adjustment in separate PRs. We'll also have a review with the PM because some aspects of the flow are not clear.