Add content collection custom IDs section

[HiDeoo]

Description (required)

As discussed on Discord, this PR adds back an equivalent of the “Defining custom slugs” section in the current version of the docs for the content collections page in the v5 docs.

The content is based on the current version but updated to match the new changes related to the Content Layer API.

A few notes:

• It works with JSON files too.
• The sentence about slug being a special, reserved property name not allowed in schema is removed, as it is not true anymore.
• Not sure about the capitalization of IDs in the title.

Related issues & labels (optional)

• Closes #
• Suggested label: 5.0.0-beta

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.