Contributions by @saivann to devel docs

Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.

The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad
Contributions by @instagibbs to devel docs

Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.

The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad
Contributions by @harding to devel docs

Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.

The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad
Fix transactions link on the devel-doc page
Small Changes To Devel Doc Based On Comments By @luke-jr & @mikehearn

_includes/guide_transactions.md:

* DOS expanded to "denial of service" to improve readability and avoid
  conflation with Disk Operating System. Change based on feedback from
  @luke-jr. Thanks!

_includes/guide_mining.md:

* Dropped assertion that `getblocktemplate` can't reuse an established
  socket. Change based on feedback from @luke-jr. Thanks!

_includes/guide_contracts.md:

* Dropped opening sentences to Contracts section, which were a holdover
  from when contracts was a subsection of Transactions. New opening
  sentence is now similar to the summary sentences which open all other
  sections. Change based on feedback from @mikehearn. Thanks!

* Deleted USA-centric example from second paragraph and merged remaining
  parts of the first two paragraphs into a single opening paragraph with
  no example. Change based on feedback from @mikehearn. Thanks!

* Removed mention of placeholder byte from multisig example. Change
  based on feedback from @mikehearn. Thanks!
Remove Assertion That Pay-To-Pubkey (Unhashed) Still Used In Coinbase Txes

_includes/guide_transactions.md:

* Assertions that pay-to-public-key (unhashed) was "used in all coinbase
  transactions" and "is more convenient" have been removed: the first
  statement because its provably untrue; the second because it's debatable
  (spending unhashed keys requires fewer bytes, so it might be more
  convenient).  Based on feedback from @TierNolan.  Thanks!
Add Comparison-Based Attacks To Reasons Not To Reuse Keys

_includes/guide_transactions.md:

* Expand the security part of the Avoiding Key Reuse subsection to also
  describe why creating more than one signature with the same private
  key might be a problem. Based on feedback from @luke-jr. Thanks!
Mention Side-Channel Attacks In Addition To Lack-Of-Entropy Attacks
Merge pull request #394 from harding/docsupdate1

Small Changes To Devel Doc Based On Comments
Merge pull request #395 from harding/docsupdate-sigsecurity

Devel Docs: Add Comparison-Based Attacks To Reasons Not To Reuse Keys
Replace P2PH by P2PKH in devel-docs
Merge pull request #400 from bitcoin/develp2pkh

Replace P2PH With P2PKH
Fix Off-By-One Error In HD Wallet Section

Discovered by /u/lifeboatz on Reddit.  Thanks!
Merge pull request #401 from bitcoin/devdoc-hd-offby1

Fix Off-By-One Error In HD Wallet Section
Fix Formula For Normal HD Key Derivation; Mention Ancestor Key Risk

_includes/guide_wallets.md:

* Fix formula given for normal child key derivation to state that public
  keys must also be provided to the HMAC hash function. This required
  updating both text and images.

* Add one-paragraph warning about ancestor key compromise when the
  ancestor extended public key is compromised along with a descended
  private key.  Update img/dev/en-hd-private-parent-to-private-child.*
  to help illustrate this warning.

en/developer-reference.md:

* Remove %include% of previously-removed file which caused new versions
  of Jekyll to die.
Add "Report An Issue" action in developer documentation toc
Improve mini_ico_report icon
Update "Report An Issue" link on mouseover (thanks @harding)
Fix toc action entries style for mobiles
Merge pull request #408 from harding/hd-ancestor-compromise

Fix Images For Normal HD Key Derivation; Mention Ancestor Key Risk