Fix Links for Hugo

[jameskerr]

I've removed the .md suffixes from the links in the docs.

I've removed the _index from the end of section pages.

We should run the link checker on this soon to see if there are any broken straggler links.

[philrz]

Hmm. I don't think that matches how I'd expected we might go about addressing that. The way things are now on this branch, when trying to browse within GitHub below docs/, pretty much all the links are now broken. For instance if you go to https://github.com/brimdata/super/blob/fix-links-for-hugo/docs/commands/_index.md and click that first link under "The super command is..." it's a 404 since the underlying file still ends in .md. Now, I'm mindful of the fact that the top priority here is how things look & feel in their final home within the docs site. But in addition to it being convenient that users could still do some light docs browsing within GitHub itself, having working links within the super repo itself is also helpful since it allows for running a markdown-based link checker while PRs are still under development or right after they're merged. And actually renaming the files themselves in the super repo to not have an .md extension would lose us a couple things, such as GitHub would no longer render the markdown (once again handy for quick browsing within the repo), as well as it being a self-documenting extension.

Maybe there's other ways to go about this, but my first instincts were to just see how Docusaurus was doing what it was doing and somehow mimic that. I don't know exactly how they accomplished it at the code level, but we always were able to keep things as they have been on the GitHub side and then when building the docs site contents they'd just morph things to drop the .md extensions while keeping the hyperlinks between pages working to match that, have things that started on the super repo as README.md files become top-level links (e.g., https://github.com/brimdata/super/blob/main/docs/commands/README.md becomes https://zed.brimdata.io/docs/next/commands) etc.