docs: Fix markdown errors, linting and spelling

[sammcj]

As per https://github.com/nicolai-wachenschwan/arc-overhang-prusaslicer-integration/pull/39#pullrequestreview-1381986935

---

Howdy,

Interesting project.

This PR fixes:

• Most of the Markdown errors and linting problems.
• Spelling mistakes.
• Slightly improves instructions around the python version.
• Set a minimum python version of 3.5 in requirements.txt.

Enjoy! 😄