Changes:
- Simplified `default.nix` and `shell.nix`
- Moved the default shell to `nix/shell.nix`
- Dropped unnecessary merge from `nix/shells.nix`
- Fixed `nix/lib/getConfig.nix` to return default on `null`
- Expanded `nix/DETAILS.md` with more info
- Added links to presentations in `nix/README.md`
- Fixed a few typos
Signed-off-by: Jakub Sokołowski <jakub@status.im>
