Draft: Docs: Extract tutorial and improve container registry metadata database docs

What does this MR do and why?

Extracts the embedded tutorial content from the container registry metadata database page into a standalone Tutorial page, and improves the main page and subpages for clarity and scannability.

Changes:

New file:

• container_registry_metadata_database_tutorial.md: New Tutorial page titled "Tutorial: Enable the container registry metadata database" that combines the Before you begin warnings, path selection (new vs existing registry), and post-import monitoring guidance into a single end-to-end walkthrough.

Main metadata database page (container_registry_metadata_database.md):

• Adds a reference to the new tutorial at the top of the "Enable the metadata database" section
• Removes the "Before you start" section (content moved to the tutorial)
• Improves the intro paragraph: fixes passive voice and run-on sentences
• Fixes minor grammar issues (Oxford comma, "may" to "can")

Subpages:

• New install: Adds a note directing existing-registry users to the correct page; improves ending sentence
• One-step import: Improves intro; adds cross-reference to three-step method; improves ending sentence
• Three-step import: Improves intro; adds cross-reference to one-step method; fixes "may" to "can"; improves ending sentence

Motivation: The main metadata database page had a tutorial-like section embedded in a reference/task page, making it harder to navigate. Extracting it as a proper Tutorial page follows GitLab docs conventions and improves the user journey.

References

• Issue: #606166
• Related MR (Geo page): !246581

Screenshots or screen recordings

Before	After
"Before you start" warnings buried in main page	Standalone Tutorial page with clear step-by-step flow
No cross-references between one-step and three-step methods	Each import page references the alternative method

How to set up and validate locally

Review the rendered Markdown at:

• doc/administration/packages/container_registry_metadata_database_tutorial.md (new)
• doc/administration/packages/container_registry_metadata_database.md
• doc/administration/packages/container_registry_metadata_database_new_install.md
• doc/administration/packages/container_registry_metadata_database_one_step_import.md
• doc/administration/packages/container_registry_metadata_database_three_step_import.md

MR acceptance checklist

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