Rename oasis-indexer to nexus

[mitjat]

Marketing directive.

Renames/changes:

• all references to github.com paths (notably, Go imports)
• the structure of docker images; all oasis-indexer path components are renamed nexus, including mount points and the name of the main binary
  • These changes are substantial, but perhaps counterintuitively, it is easier to change most occurrences of oasis-indexer with nexus, rather than being very selective and changing only some, e.g. keeping oasis-indexer in most places but renaming only the binary (a "requirement"). It will hopefully also reduce confusion for readers to mostly have a single name used consistently.
• most usages of "indexer" everywhere in the codebase
• adds an intro paragraph to top-level README

Does NOT rename/change:

• The DB usernames and dbnames in our various docker setups. Many contain "indexer". While perfectly doable to rename, it's brittle. We can do it later if we want.
• The docker image repo, or the references to it. That one remains as oasislabs/oasis-indexer (that's Labs!)

[tjanez]

Great work, thanks!

[Andrew7234]

(no-op) I'd prefer oasis-nexus over nexus since we refer to it as Oasis Nexus in the docs. It'd also be consistent with Oasis Explorer and how most of our services historically were prefixed with oasis-. But since this rename is requested by marketing it's not up to us so I'll just lodge my objection here for posterity and we can leave as-is.

[mitjat]

I actually prefer nexus here because it's more of a token/identifier, and in that role, we never use oasis-nexus (with a hyphen); for consistency and simplicity, I like that we always have one or the other.
We mostly use "Oasis Nexus" in external docs when there is little external context and we want to additional qualify the name. We also use just "Nexus" in externally facing docs plenty of time.