docs: update collections.md to document H1 heading requirement for collection.md files

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**Component:** Documentation

**Bug Description**

`docs/customization/collections.md` does not mention that `*.collection.md` files must begin with a top-level H1 heading. This requirement was enforced in commit [`42cc137`](https://github.com/microsoft/hve-core/commit/42cc13775e90f84332846ea47a46045a8ae983d4) (#1372), which removed the markdownlint exclusion for `collections/*.collection.md` and updated all existing collection files to add MD041-compliant H1 headings.

Additionally, `scripts/plugins/Modules/PluginHelpers.psm1` now strips this H1 heading when injecting collection content into plugin READMEs (to avoid duplicate titles). This stripping behavior is undocumented.

**Expected Behavior**

Step 3 in the "Creating a Collection" section of `docs/customization/collections.md` should note:
- The `*.collection.md` file must begin with a top-level H1 heading (enforced by MD041 markdownlint rule).
- The H1 is stripped automatically during plugin generation; it should contain the human-readable collection name.

**Steps to Reproduce**

See PR #1372 (commit `42cc137`), which introduced:
- Enforcement of MD041 on `collections/*.collection.md`
- H1 stripping in `scripts/plugins/Modules/PluginHelpers.psm1`

**Additional Context**

- Documentation file: `docs/customization/collections.md` — "Creating a Collection" section, step 3
- Code file: `scripts/plugins/Modules/PluginHelpers.psm1` — `New-PluginReadmeContent` function
- Code file: `scripts/extension/Prepare-Extension.ps1` — `New-CollectionReadme` function (wraps generated H3 sections under H2 for MD001 compliance)

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Start with step 3 in docs/customization/collections.md, then read New-PluginReadmeContent in scripts/plugins/Modules/PluginHelpers.psm1 and New-CollectionReadme in scripts/extension/Prepare-Extension.ps1. Update the documentation to cover the required H1, its collection name, and stripping during generation; verify the file satisfies MD041 and the documented behavior matches these entry points.

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