Fix incorrect usage of 'script' rather than 'scriptPubKey'

This is causing quite a bit of confusion, for example by people looking
in the Bitcoin Core sourcecode and seeing the term 'scriptPubKey'
instead.
Replace misleading 'return' terminology with 'push'

Saying opcodes return values gives the impression the script is being
returned.
Small fixes to scriptPubKey/scriptSig pull

* Small grammar fixes.

* Lower case #term-scriptPubKey as all our other anchor links are
  lower case

* Replace script/scripts with scriptPubKey/scriptPubKeys in
  _autocrossref.yaml. (Fixes `make test` errors from broken
  auto-crossref links.)
Use Terms "Pubkey Script" And "Signature Script"

This modifies commits provided by @petertodd to use the terms "pubkey
script" and "signature script" instead of other terms.

* Rename "scriptPubKey" and "output script" to "pubkey script"
  (suggested by @luke-jr). We leave a token "scriptPubKey" at the point
  where we define pubkey script so that searchers can find it.

* Rename "scriptSig" to "signature script" (suggested by @luke-jr). We
  also leave a token "scriptSig" at this definition point.

* Rename "redeemScript" to "redeem script"

* Defined ECDSA on secp256k1 curve as the crypto used in the Transaction
  section and added references to secp256k1 private/public keys and
  signatures.

* Removed "The Parts Of A Transaction" illustration by commenting it out
  in the HTML. Shoehorning the pubkey/signature script terms into this
  image was becoming difficult, and I'm not very fond of that
  illustration anyway. I'll see if I can think of a nicer replacement
  illustration for some point in the future.

* Add a short paraphrased version @petertodd's description of scripts as
  generalized crypto.

* Updated all the illustrations which referred to either pubkey scripts
  or signature scripts to use these terms.
Merge branch 'script-terms'