General documentation updates

- We now have several developer-focused docs, so create an index page
  for them.
- Rework several docs to fit into the index structure.
- Experiment with code highlighting; tweak the CSS slightly to make
  it slightly nicer to look at.

Change-Id: I4185a18f84fa0764745ca7a3148276064a3155c6
Signed-off-by: Chris Luke <chrisy@flirble.org>

extras/libmemif/docs/buildinstructions_doc.md

@@ -1,4 +1,4 @@
-## Build Instructions
+## Build Instructions    {#libmemif_build_doc}
 
 Install dependencies
 ```
@@ -49,6 +49,7 @@ commands:
 	ip-set <index> <ip-addr> - set interface ip address
 	rx-mode <index> <qid> <polling|interrupt> - set queue rx mode
 ```
+
 #### Examples
 
-Once the library is build/installed, refer to [Examples](../examples/README.md) and [Getting started](GettingStarted.md) for additional information on basic use cases and API usage.
+Once the library is built/installed, refer to @ref libmemif_examples_doc and @ref libmemif_gettingstarted_doc for additional information on basic use cases and API usage.

extras/libmemif/docs/gettingstarted_doc.md

@@ -1,8 +1,8 @@
-## Getting started
+## Getting started    {#libmemif_gettingstarted_doc}
 
 #### Concept (Connecting to VPP)
 
-For detailed information on api calls and structures please refer to [libmemif.h](../src/libmemif.h)
+For detailed information on api calls and structures please refer to @ref libmemif.h.
 
 1. Initialize memif
    - Declare callback function handling file descriptor event polling.
@@ -152,7 +152,8 @@ if (err != MEMIF_ERR_SUCCESS)
 
 #### Example app (libmemif fd event polling):
 
-- [ICMP Responder](../examples/icmp_responder/main.c)
+- @ref extras/libmemif/examples/icmp_responder
+
 > Optional argument: transmit queue id.
 ```
 icmpr 1
@@ -162,11 +163,14 @@ icmpr 1
 
 #### Example app:
 
-- [ICMP Responder custom fd event polling](../examples/icmp_responder-epoll/main.c)
+ICMP Responder custom fd event polling.
+
+- @ref extras/libmemif/examples/icmp_responder-epoll
 
 #### Example app (multi-thread queue polling)
 
-- [ICMP Responder multi-thread](../examples/icmp_responder-mt/main.c)
+ICMP Responder multi-thread.
+- @ref extras/libmemif/examples/icmp_responder-mt
 
 > Simple example of libmemif multi-thread usage. Connection establishment is handled by main thread. There are two rx queues in this example. One in polling mode and second in interrupt mode.