:memo: Add user guide file to docs

[dalonsoa]

Description

Adds the existing user_guide.md file to the MkDocs documentation, so it is rendered alongside everything else. This file is just a placeholder. Populating the guide will be done in #350 .

Fixes #346

Type of change

• [ ] New feature (non-breaking change which adds functionality)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Documentation (non-breaking change that adds or improves the documentation)
• [ ] Optimisation (non-breaking, back-end change that speeds up the code)

Key checklist

• [x] Pre-commit hooks run successfully (pre-commit run -a)
• [ ] All tests pass (pytest)
• [ ] The documentation builds without warnings (mkdocs build -s)
• [ ] Check the GUI still works (if relevant)
• [ ] Check the code works with actual hardware (if relevant)
• [ ] Check the pyinstaller-built executable works (if relevant)

Further checks

• [ ] Code is commented, particularly in hard-to-understand areas
• [ ] Tests have been added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.99%. Comparing base (f014b51) to head (550c79f). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #679 +/- ## ======================================= Coverage 84.99% 84.99% ======================================= Files 69 69 Lines 3459 3459 ======================================= Hits 2940 2940 Misses 519 519 ```

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