Rework discovery-ec2 docs (elastic#41630)

This commit reworks and clarifies the docs for the `discovery-ec2` plugin:

- folds the tiny "Getting started with AWS" into the page on configuration
- spells out the name of each setting in full instead of noting the
  `discovery.ec2` prefix at the top of the page.
- replaces each `(Secure)` marker with a sentence describing what that means in
  situ
- notes some missing defaults
- clarifies the behaviour of `discovery.ec2.groups` (dependent on `.any_group`)
- clarifies what `discovery.ec2.host_type` is for
- adds `discovery.ec2.tag.TAGNAME` as a (meta-)setting rather than describing
  it in a separate section
- notes that the tags mentioned in `discovery.ec2.tag.TAGNAME` cannot contain
  colons (see elastic#38406)
- clarifies the EC2-specific interface names and what they're for
- reorders and rewords the recommendations for storage
- expands on why you should not span a cluster across regions
- adds a suggestion on protecting instances against termination during scale-in
- reformat to 80 columns where possible

Fixes elastic#38406