doc: Add bootstrap context to tutorial

[zeel2104]

Description

Adds brief context to the “Initialise your cluster” section of the MicroCeph tutorial, explaining what bootstrapping does at a high level before the user runs microceph cluster bootstrap.

This helps new users understand that bootstrap creates the initial cluster configuration and keys, registers the first node, and starts the core Ceph services.

Fixes: canonical/open-documentation-academy#352 (comment)

Type of change

• Documentation update (change to documentation only)

How has this been tested?

• Ran git diff --check successfully.
• Attempted a local Sphinx build with sphinx-build --fail-on-warning --keep-going -b dirhtml . _build -c . -d _dev\.doctrees -D llms_txt_enabled=0, but it could not complete because the local environment is missing the canonical_sphinx Python package.

Contributor checklist

Please check that you have:

• self-reviewed the code in this PR
• added code comments, particularly in less straightforward areas
• checked and added or updated relevant documentation
• added or updated HTML meta descriptions for any new or modified documentation pages (see #643)
• verified that page title and headings accurately represent page content for new or modified documentation pages
• checked and added or updated relevant release notes
• added tests to verify effectiveness of this change

[skoech]

Thank you for your prompt contribution, @zeel2104!

Since it's your first time contributing, you will need to sign the Canonical Contributor Licence Agreement (CLA). If you already have, it may take some time (up to a day) to reflect.