Omnitrace docs refactoring

[amd-jnovotny]

This PR contains the changes necessary to integrate Omnitrace into the ROCm documentation environment for release 6.2. It involves the following tasks:

• Moving the documentation out of source/docs and into the main docs directory.
• Moving some existing pages into subdirectories to conform to the ROCm categories of tutorials, how to, conceptual background, and reference materials. Some existing pages might be split up into multiple pages in the new arrangement, while other pages might combine material from two existing pages.
• Reformatting the material from Markdown to rst.
• Changing a few page and section titles for greater consistency with other ROCm documentation hubs.
• Making some behind-the-scenes changes to accommodate our Read the Docs publishing pipeline and conform to the ROCm documentation standard look-and-feel.

Commits will be done in stages. The initial commit includes the main index file and a slightly rewritten "What is Omnitrace?" page, along with some scaffolding changes. New pages will be pushed regularly as work proceeds.

[samjwu]

closing and reopening to trigger RTD pr builds

[samjwu]

todo: generate Doxyfile from the cmake

[amd-jnovotny]

The content is basically ready for review with a few caveats:
-- The API structure and links need to be finalized. Omnitrace uses a different structure than rocDecode and others, which provide additional formatting.
-- We need to decide on a redirect strategy for the existing content
-- The old docs files remain in source/docs in case anyone wants to cross-reference them. They will eventually be removed.
-- I intend to do a final sweep through the docs for formatting and typos.

[dgaliffiAMD]

I'm still working through reviewing all of the "How-Tos". Here is the latest batch.