Add basic style guide

Author: msimberg

STYLE.md

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+# Style Guide
+
+This guide contains general guidelines for how to format and present documentation in this repository.
+They should be followed for most cases, but as a guideline it can be broken, _with good reason_.
+
+The documentation is written using [Material for MkDocs](https://squidfunk.github.io/mkdocs-material/).
+
+## General Formatting
+
+- Format paragraphs with one sentence per line for easier diffs.
+- Leave a space before and after headings.
+
+## Headings are Written in Title Case
+
+Use [title case](https://en.wikipedia.org/wiki/Letter_case#Title_case) for headings, meaning all words are capitalized except for minor words.
+
+## Avoid Nesting Headings too Deep
+
+Nesting headings up to three levels is generally ok.
+
+## Unordered and Ordered Lists
+
+Write lists as proper sentences.
+Separate the items simply with commas if each item is simple, or make each item a full sentence if the items are longer and contain multiple sentences.
+
+1. The first item can look like this,
+2. The second like this, and
+3. The third item like this.
+
+## Using Admonitions
+
+Aim to include examples, notes, warnings using [admonitions](https://squidfunk.github.io/mkdocs-material/reference/admonitions/) whenever appropriate.
+They stand out better from the main text, and can be collapsed by default if needed.
+
+!!! example "Example one"
+    This is an example.
+    The title of the example uses [sentence case](https://en.wikipedia.org/wiki/Letter_case#Sentence_case).
+    
+??? note "Collapsed note"
+    This note is collapsed, because it uses `???`.
+
+[](){#ref-style-references}
+## References
+
+Add references to headings to allow easier cross-referencing from other sections.
+Prefix the reference name with `ref-`, followed by at least one level of identifier for the page itself.
+Finally, add an identifier for the heading itself.
+[This section][ref-style-references] uses the identifier `ref-style-references`.