Edit pass for consistency

[bamurtaugh]

Complete the edit pass mentioned in devcontainers/spec#293 (comment)

Main changes:

• Refer to the spec as "Development Container Specification"
  • All caps
  • Singular "Container" rather than "Containers" (plural feels awkward)
• The term "dev container" shouldn't be capitalized on its own
• Signify devcontainer.json is a file type through `
• Features
  • Entire name is "Dev Container Features" (note the caps)
  • On their own, "Features" (note the caps)
• Templates - same as Features
• Refer to CLI as "Dev Container CLI" (note the caps)
• Just use bolding for emphasis sprinkled throughout sections, rather than try to use it to always bold certain terms (this felt inconsistent and confusing)
• Various other typos or formatting issues (i.e. the new CodeSandbox supporting tools section had some table issues)

I'll similarly review https://github.com/devcontainers/spec to provide a similar edit pass, following the guidance established here.

[samruddhikhandale]

Than you for taking the time in contributing this PR 👏
It looks great, left some minor comments!

[bamurtaugh]

Made one more change and resolved some comments! @samruddhikhandale if you're able to give a quick re-review and approval, that'd be fantastic 🚀 Thank you!

[samruddhikhandale]

⚡