WIP: Update Website Using NIDM Acronym

There are only a couple commits here (some of the changes were incorporated into incf-nidash/nidm#103), but the goal of this PR is to update the website and specs for the upcoming INCF Review that @satra will be participating in.

I think the website and NIDM-Overview specs look good, but it would be great if folks could review and send me any changes (preferably as a PR). 

NIDM-Results still needs to be updated to use the new NIDM-* convention throughout, perhaps @cmaumet can take a look at that.

- [x] Verify website is consistently using NIDM
- [x] Verify NIDM-Overview spec is consistently using NIDM
- [x] Verify NIDM-Results spec is consistently using NIDM
- [ ] Finish draft of NIDM-Primer (?)
- [ ] Update NIDM Layer Cake per incf-nidash/nidm#102 (?)

Feel free to check things off if you've verified!