Rdoc enhancements for psych.rb

[BurdetteLamar]

This PR concerns primarily the high-level interface:

• Added introductory material, with links to the spec and Ruby cookbook at yaml.org.
• Modified high-level API discussion, with new examples and links to the relevant methods.
• Added sections on options, which the methods can link to.
• Enhanced doc for load and dump methods:
  • load
  • load_file
  • load_stream
  • dump
  • dump_stream

This PR is larger than I would have liked, but the high-level API concepts are intertwined, and the their doc needs to be modified all at once.

[BurdetteLamar]

@hsbt, I've requested a review from you because you're the frequent contributor in recent history. Let me know if you'd like me to request someone else.

[BurdetteLamar]

@hsbt, should I ask someone else to review?

[BurdetteLamar]

@hsbt, I think this is ready for review.

[olleolleolle]

I like this change, in that it strengthens the "general section" which provides a complete narrative, for the learner/reader.

[olleolleolle]

Looks ✨ good ✨ to me!

[BurdetteLamar]

@hsbt, @olleolleolle has reviewed and approved this PR. Ok to merge? I'm reluctant without hearing from you.

[BurdetteLamar]

@tenderlove, would appreciate a blessing from you or @hsbt before merging this.

[hsbt]

@BurdetteLamar You ignored our feedback at ruby/ruby#3506 (comment)

Because I couldn't/will not approve your pull-request.

[BurdetteLamar]

@tenderlove, you are a maintainer here? Can you (or someone) review this?