image/spec: minor fixes and touch-ups in markdown files

[thaJeztah]

• remove some trailing commas, which made the JSON invalid (some of these were fixed in the 1.2 spec, but not in older versions).
• synchronise some formatting / phrasing between versions, to make them easier to compare.
• remove non-breaking spaces (NBSP) in example outputs, and replace them with regular spaces.

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

After this, I'll have a look at back-filling history, per #46363 (comment)

I did a check on history:

• the image spec v1.0.0 was added in Adds Docker Image v1 Spec Documention #9560

• ONBUILD was added in New build instruction: ONBUILD defines a trigger to execute when extending an image with a new build #3254

  • This was before image spec v1.0.0, so "overlooked" ?

• ArgsEscaped was added in 9db5db1 / Prevent double escaping of Dockerfile commands on Windows #17851

  • First release: v1.10.0-rc1
  • This was after v1.0.0, but before v1.1.0; https://github.com/microsoft/docker/tree/9db5db1b94bc1000d151315851096dcc6cd3512d/image/spec (so v1.1.0 should be amended)
  • Adopted by OCI in Add ArgsEscaped field to image config opencontainers/image-spec#892

• SHELL was added in b18ae8c / Builder shell configuration #22489

  • First release: v1.12.0-rc1
  • This was after v1.1.0, but before v1.2.0; https://github.com/microsoft/docker/tree/b18ae8c9ccc2eb6cf8aa947f25eb6f1d20089776/image/spec (so a "v1.2.1" ?)

So, in order of changes;

• Add Onbuild to v1.0.0, v1.1.0, and v1.2.0 ("bugfix")
• Add ArgsEscaped to v1.0.0 and make it version v1.0.1
• Add ArgsEscaped to v1.1.0 ("bugfix")
• Add ArgsEscaped to v1.2.0 ("bugfix")
• Add Shell to v1.1.0 and make it v1.1.1
• Add Shell to v1.2.0 ("bugfix")

We can REMOVE ArgsEscaped from the spec in v1.3.0, as "superseded by OCI"