Concepts section overhaul (mirror project)

[noramullen1]

• Previously, the Pinot OSS 'concepts' page was an overview of the components. However, we have a separate section and pages for all the components. I moved all the conceptual info on the components to their respective pages.
• I added a concepts section to OSS with the same subpages/"concepts" as in the Pinot section of the ST docs, because that info was missing from the OSS docs.
• Also added a redirect.

[noramullen1]

Addresses the following Jira issues:

• https://startree.atlassian.net/browse/DOCS-312
• https://startree.atlassian.net/browse/DOCS-294
• https://startree.atlassian.net/browse/DOCS-295
• https://startree.atlassian.net/browse/DOCS-296