Add spell checker section to contributing guide (#198)

Author: msimberg

docs/contributing/index.md

@@ -63,6 +63,37 @@ You can get some hints of whom to contact by looking at [CODEOWNERS](https://git
     To minimise the overhead of the contributing to the documentation and speed up "time-to-published-docs" we do not have a formal review process.
     We will start simple, and add more formality as needed.
 
+### Spell checker
+
+A spell checker workflow runs on all PRs to help catch simple typos.
+If the spell checker finds words that it considers misspelled, it will add a comment like [this](https://github.com/eth-cscs/cscs-docs/pull/193#issuecomment-3056795496) to the PR, listing the words that it finds misspelled.
+
+The spell checker isn't always right and can be configured to ignore words.
+Most frequently technical terms, project names, etc. will not be in the dictionaries.
+There are three files used to configure words that get ignored:
+
+- `.github/actions/spelling/allow.txt`:
+  This is the main file for whitelisting words.
+  Each line of the file contains a word that is ignored by the spell checker.
+  All lowercase words are matched with any capitalization, while words containing at least one uppercase letter are matched with the given capitalization.
+  Using the capitalized word is useful if you always want to ensure the same spelling, e.g. for names.
+<!--begin no spell check-->
+- `.github/actions/spelling/patterns.txt`:
+  This file is used to ignore words that match a given regular expression.
+  This file is useful to ignore e.g. URLs or markdown references.
+  Words that have unusual capitalization may also need to be added to this file to make sure they are ignored.
+  For example, "FirecREST" is normally recognized as two words: "Firec" and "REST", and adding "FirecREST" to `allow.txt` will not ignore the word.
+  In this case it can be ignored by adding it to `patterns.txt`
+<!--end no spell check-->
+- `.github/actions/spelling/block-delimiters.txt`:
+  This file can be used to ignore words between begin- and end markers.
+  For example, code blocks starting and ending with `` ``` `` are ignored from spell checking as they often contain unusual words not in dictionaries.
+  If adding words to `allow.txt` or `patterns.txt`, or ignoring blocks with `block-delimiters.list`, is not sufficient, you can as a last resort use the HTML comments `<!--begin no spell check-->` and `<!--end no spell check-->` to ignore spell checking for a larger block of text.
+  The comments will not be rendered in the final documentation.
+
+Additionally, the file `.github/actions/spelling/only.txt` contains a list of regular expressions used to match which files to check.
+Only markdown files under the `docs` directory are checked.
+
 [](){#ref-contributing-guidelines}
 ## Guidelines