Align page and file names

It has long annoyed me that our page names and filenames don't
match.  I open specification-links.md when I mean to open
documentation.md probably half the time at least.

* Update "gems" to "plugins" to get jekyll to stop complaining
* Add the redirect plugin
* Rename documentation.md to specification.md and add a redirect
* Change the title of implementations.md to Implementations

Remove most remaining links to ".md" URIs and standardize on
".html" which seems to be what we've been going for.  I think
there are still example links with ".md" but I did not want
to touch the examples at all for now.

_config.yml

@@ -27,7 +27,7 @@ markdown: kramdown
 theme: minima
 
 header_pages:
-- documentation.md
+- specification.md
 - examples.md
 - implementations.md
 
@@ -36,5 +36,6 @@ exclude:
 - Gemfile
 - node_modules
 
-gems:
+plugins:
   - jekyll-relative-links
+  - jekyll-redirect-from

implementations.md

@@ -1,6 +1,6 @@
 ---
 layout: page
-title: Software
+title: Implementations
 permalink: /implementations.html
 ---
 

index.md

@@ -4,7 +4,7 @@ title: JSON Schema
 permalink: /
 ---
 
-***Draft-07 has been [published](documentation.md)!***
+***Draft-07 has been [published](specification.html)!***
 {: style="color:gray; font-size: 150%; text-align: center;"}
 
 **JSON Schema** is a vocabulary that allows you to **annotate** and **validate** JSON documents.
@@ -86,9 +86,9 @@ More
 
 Interested? Check out:
 
--   the [specification](documentation.md)
+-   the [specification](specification.html)
 -   some [examples](examples.md)
--   the growing list of [JSON (Hyper-)Schema software](implementations.md)
+-   the growing list of [JSON (Hyper-)Schema software](implementations.html)
 
 We encourage updating to the latest specification, as described by the draft-07 meta-schemas.  However, if you are still using draft-04, you may be interested in:
 -   this [excellent guide](http://spacetelescope.github.io/understanding-json-schema/) for schema authors, from the [Space Telescope Science Institute](http://www.stsci.edu/)

specification-links.md

@@ -5,7 +5,7 @@ layout: page
 
 <!-- Links on this page should be immutable - none of them should go to `/latest`, etc. -->
 
-You can find the latest released draft on the [Specification](/documentation.md) page.  Older drafts are expired, but may be of historical interest.
+You can find the latest released draft on the [Specification](/specification.html) page.  Older drafts are expired, but may be of historical interest.
 
 **A note on draft naming and numbering:**  
 IETF Internet-Drafts (I-Ds) are named with the editor's name and a sequential number which resets with each new editor.  Meta-schemas are numbered sequentially.  Additionally, drafts 00-03 used one document for all three current specs.  Most people find it easier to remember the sequential meta-schema numbers, so those are used throughout the site.

documentation.md → specification.md

@@ -1,5 +1,7 @@
 ---
 layout: page
+redirect_from: "/documentation.html"
+permalink: /specification.html
 title: Specification
 ---