Better section naming and organization

[netsettler]

• Remove a useless link to the "Overview" on the first page.
• Make the heading for "Basic Setup" to be "Installing Prerequisites", since that naming is more standard.
• Rename the "Getting Started" option to more standard "Using submitr", since getting started is ambiguous between installation and usage.
• Add an "Implementation of submitr" heading on the implementation part to make it clear to end users they don't need to look at this.
• Reorganize to make experimental rclone support not be the first thing that you see in this doc, since non-experimental stuff needs to be first.
• Make rclone section not pretend to tell you about awscli in the heading, since the actual text barely mentions awscli.
• Light editing on the opening of the section about rclone to make the motivational part clearer.

NOTE TO REVIEWERS:

You can preview this by downloading the repo and doing

make preview-locally

That will build an html set in docs/html/ (which I've made invisible due to .gitignore) and will open a browser to view it.

[drio18]

Consider dropping this section from the documentation entirely for the time being. I don't believe we're looking to utilize rclone in this fashion for SMaHT (via a publicly available bucket), though I know we're looking at the possibility of integrating the tool at some point.

[netsettler]

Yeah, that's fine with me. This section is available in the CGAP documentation. Maybe I can rearrange the CGAP documentation in this same way and just leave it there. Did it make more sense in that context, or ought it be removed there as well? For tentative removals, I just rename files to whatever.DISABLED and I can do that rather than deletion if you're uncertain as to whether we'd want to later add it back in one or the other context).

[drio18]

It made sense in the context of CGAP, so I think rearranging there is a good idea. Disabling for this repo is best for now to prevent possible miscommunication with SMaHT folks until we have settled the issue.

[netsettler]

Great. That's what I'll do then.