Improve formatting of Linux documentation

[juanruitina]

Some rather minor changes to the Linux page, for clarity:

• Fix the formatting on the first numbered list
• Add headings for better scannability
• Explain Orca is the default screen reader in popular distributions
• I suggest to be explicit about the command to run to change directory in the Orca instructions. I think we shouldn't assume all users are confortable with the terminal; it's enough that we ask them to use it.

(Full disclosure: I am UX designer for Ubuntu Desktop)

I license this contribution under the terms set out in the Unlicense license.

I went through the process just yesterday, it worked well. I'm delighted with the new voice :)

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