[Observability:Streams] Add page-level description to Streams landing page

[couvq]

Closes https://github.com/elastic/streams-program/issues/1106

Summary

This PR adds a page level description to the streams landing page along with a link to streams documentation.

[infra-vault-gh-plugin-prod]

Pinging @elastic/obs-onboarding-team (Team:obs-onboarding)

[flash1293]

Code wise this looks good, I'm not sure about the screen use though - this eats into the vertical screen space which is scarce already with numerous headers and whatnot.

Also we have this callout there for first time users:

@patpascal @boriskirov wdyt?

[3kt]

@flash1293 this is good feedback - I think we're reclaiming some vertical space once the chrome changes are through.
Keen on getting designers' opinion though ;) - I'd rather have "continuous clarity", and alignment with other pages (see https://github.com/elastic/streams-program/issues/1106) even if it means one-time pain for users when they dismiss the tour.

A middle ground might be to make the tour banner slicker? For example the "view docs" would be redundant.
If we have to pick one, I'd rather be in favor of the description, as I've be bonked on the head for my appetite of tours 😆

[kibanamachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 1838d4d

Failed CI Steps

• FTR Configs #111

Test Failures

• [job] [logs] FTR Configs #111 / integrations When in the Fleet application and on the Endpoint Integration details page should show the Trusted Apps page when link is clicked

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
streamsApp	2.0MB	2.0MB	+426.0B

History

• 💚 Build #443122 succeeded 4ae538a
• 💚 Build #442083 succeeded c21193a
• 💛 Build #440661 was flaky 9940dc3
• 💛 Build #440135 was flaky ad71b80

[flash1293]

LGTM, lets start with this for now and follow up