Small fixes to 'data streams' section

[kilfoyle]

This is just a small follow-up PR to the earlier #490

It:

• Removes the "Index types" page (all content in that page is already captured in the "index basics" page that Wajiha assembled
• Moves the "Logs data stream" page to be parallel with "Time series data stream", rather than nested under it (a mistake from my earlier PR).

I also planned to add a note about "data streams" to the "Index basics" page, but I see Wajiha already has that one covered. :-)

Closes: https://github.com/elastic/docs-projects/issues/324

[colleenmcginnis]

Should we add something about data streams to https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/508/manage-data/data-store now that it is at the same level as the other links referenced on this page?

[kilfoyle]

Should we add something about data streams to https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/508/manage-data/data-store now that it is at the same level as the other links referenced on this page?

Good idea @colleenmcginnis
I've added as shown:

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