Docs: Add identifier to each Markdown file under docs

[nastra]

I noticed that https://iceberg.apache.org/docs/latest/spark-configuration/ isn't showing up under the Spark section because Spark's configuration page and the Table configuration page were both using title: "Configuration" and only the first one wins and shows up in the main menu under Docs.

In order to not cause such collisions with any other markdown files that show up in the main menu, I think the best approach is to define an identifier for each one of them.

See also https://gohugo.io/content-management/menus/#properties-front-matter for some additional details on Hugo's identifier and when it is required.

[Fokko]

Thanks, this makes sense to me

[nastra]

thanks for the reviews @Fokko and @singhpk234. I'll merge this so that we can ship it with 1.3.0