docs: full copy-edit of DAG page

[agilgur5]

Motivation

started off by just improving the fail fast section, which did not quite line up with the style guide

• then ended up modifying the rest of the page, as it was fairly short

Modifications

• structural improvements:

  • add a heading for the "Fail Fast" section
    • it has its own paragraph, should have its own section
      • especially as it's a bit aside from just using DAGs; it's a specific feature (and was added as such)
  • add a section with a link to "Enhanced Depends" because this is a very commonly asked user question
    • we direct users to the "Enhanced Depends" feature already when they ask if they can do something; let's get ahead of that by directing in DAG page already
  • add a link to the steps page when it is mentioned

• clarifications:

  • "This" -> "DAGs" -- be more specific about the subject for clarity
  • "steps", "nodes" -> "tasks" -- that is what the field is called in a DAG template

• stylistic changes:

  • 1 markdown line per sentence
  • use present tense where possible, per k8s style guide
  • significantly simplify the fail fast section, per k8s style guide
  • no need to have fail fast in backticks as it's not a code reference
    • do add backticks around the failFast field, as that's the specific keyword used in the YAML
      • remove "FailFast" from .spelling as words with backticks are ignored

• add the 1 sentence per line preference that I mentioned above and use myself to the docs styleguide

  • this way, diffs to sentences are limited to a specific line, not an entire paragraph
  • often there are no conventions for how to split lines, so a paragraph can be on multiple lines, some of which have multiple sentences, others of which are one sentence, and others which are not even a full sentence
    • we don't need to follow line width code conventions for markdown as it is prose, not code
    • we do need some convention for consistency
  • a paragraph on a single line is a bit harder to read in an editor in monospace font etc as well, IMO

Verification

make docs-spellcheck passes

[agilgur5]

Fixed merge conflict with .spelling changes from #11814 (two removals were right next to each other, both are removed now)