Update Agent Builder landing page and get-started

[charlotte-hoblik]

This PR:

• Rewrites the landing page opening paragraph
• Name the default Elastic AI Agent in the get-started
• Add a tip about cloning the default agent as a starting point for custom agents

Context

These changes are the result of reviewing PM feedback on the Agent Builder
landing page (elastic-agent-builder.md) and get-started page (get-started.md).

Connected to: https://github.com/elastic/docs-content-internal/issues/855

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/ai-features/agent-builder/get-started.md
• explore-analyze/ai-features/elastic-agent-builder.md

[github-actions]

Vale Linting Results

Summary: 1 warning, 1 suggestion found

⚠️ Warnings (1)

File	Line	Rule	Message
explore-analyze/ai-features/agent-builder/get-started.md	43	Elastic.QuotesPunctuation	Place punctuation inside closing quotation marks.

💡 Suggestions (1)

File	Line	Rule	Message
explore-analyze/ai-features/agent-builder/get-started.md	115	Elastic.Clone	Use clone only when referring to cloning a GitHub repository or creating a copy that is linked to the original. Often confused with 'copy' and 'duplicate'.

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[leemthompo]

@charlotte-hoblik as we discussed with Deepti, let's refactor here and remove the tip:

Something like this with a quick ejector seat for people who already have data in ES.

Then 2 broad buckets:

• Use sample data
• BYO data

If you already have data in Elasticsearch, skip ahead to [next step](link).

If you need to add data first, choose the path that fits:

**Use sample or synthetic data**
- Load [sample data](link) in Kibana.
- Generate synthetic financial data using this [Python tool](link)
  (requires your Elasticsearch URL and an API key).
- Follow the [Elastic Agent Builder API tutorial](link), which includes
  sample book data.

**Bring your own data**
- [Upload a file](link) through Kibana.
- For search-specific guidance, see [Ingest for search use cases](link).
- For a broader overview, see [Ingest: Bring your data to Elastic](link).

Make sure to link to the specific docs pages for the file upload and sample data, for example.

BTW it doesn't make sense to talk about the "Kibana home page" because this page must cater of all solution/project types and we can't assume users are in the ES UI.

[leemthompo]

Let's not talk about the "Kibana home page", it's too confusing on many levels