docs: move "Reserved binding keys" to "Reference guides"

[bajtos]

Also remove "Overview" heading from the page. I feel the heading is redundant when there are no other headings. Before this change, the page was rendered with a single-item Table of Contents at the top, which looked weird to me. After this change, the pages is rendered as a single block with the page title as the only heading.

This is a follow-up to #5718, see also #5549.

Checklist

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• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

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[raymondfeng]

Let's capitalize the first letter to be consistent with our urls.

 /doc/en/lb4/Reference-reserved-binding-keys.html

[bajtos]

I would like to transition our URL scheme to all-lowercase names and eventually to nested paths (/reference/reserved-binding-keys.html) to match the directory layout in docs/site.

I encountered some issues when using nested paths, so I am using - instead of / for now:

reference/reserved-binding-keys.md -> reference-reserved-binding-keys.html

We are already following the all-lowercase slug style in migration guide, see e.g. here:

https://github.com/strongloop/loopback-next/blob/ae6427322451c914ae54f44dbb656981e7fbbb81/docs/site/migration/datasources.md#L7

[bajtos]

Let's discuss in #5798

[raymondfeng]

Ditto.

[dhmlau]

LGTM once @raymondfeng's comment is addressed. Thanks.

[bajtos]

Based on discussion with @raymondfeng, we will stick with Sentence-style.html to land my PRs and then normalize URL slugs afterwards, as this part may take a bit longer to reach consensus.