Tidy repository structure

Problem

Currently, the repository contains a lot of files that are specific to building a website from the decisions. If we don't plan to do this, can we remove these files and make the repository easier to navigate.

Proposed Solution

My suggestion would be to remove the following files:

- _config.yml
- index.md
- Gemfile
- Gemfile.lock
- decisions/.markdownlint.yml
- decisions/.index.md

I would then move the decisions out of the decisions folder and just have them in root. At this point the directory structure would just be:

- 0001-mission-statement.md
- 0002-scope-of-emmet-models.md
- 0004-planning-of-developments.md
- LICENSE
- README.md
- bylaws.md
- chairperson.md

Which to me is much cleaner and more obvious to readers.

Alternatives

If we plan to actually build a website at some point then this can be closed.

materialsproject / foundation