Make repository more self-contained plus API section reorg (#3)

* Change URL from repository to website

* Add API PDFs, restructure API section

Author: szarnyasg

README.md

@@ -52,32 +52,24 @@ A straw man proposal for the GraphBLAS can be found
 [here](http://gauss.cs.ucsb.edu/~aydin/combblas-r2.pdf)
 
 Gabor Szarnyas maintains a list of [GraphBLAS
-pointers](https://github.com/szarnyasg/graphblas-pointers) with lots
+pointers](https://graphblas.org/graphblas-pointers) with lots
 of tutorial material.
 
 
 # Application Program Interface (API)
 
-Version 1.0 (provisional) of the C language API is released on May
-29, 2017 at the GABB workshop
-[here](http://graphanalysis.org/workshop2017.html).
-
-Version 1.1.0 (provisional) is released on November 14, 2017.
+## Current versions
 
-Version 1.2.0 (final) is released on May 18, 2018. It can be
-downloaded from
-[here](http://people.eecs.berkeley.edu/~aydin/GraphBLAS_API_C_v12.pdf). This
-is only available for archival purposes, you should use the latest
-version 1.3.0 below.
+* [GraphBLAS C API, version 1.3.0](/docs/GraphBLAS_API_C_v1.3.0.pdf) (September 25, 2019)
+* [GraphBLAS C API, version 2.0.0](/docs/GraphBLAS_API_C_v2.0.0.pdf) (November 15, 2021)
 
-Version 1.3.0 (final) is released on September 25, 2019. It can be
-downloaded from
-[here](http://people.eecs.berkeley.edu/~aydin/GraphBLAS_API_C_v13.pdf).
-
-Version 2.0.0 (final) is released on November 15, 2021. It can be
-downloaded from
-[here](https://github.com/GraphBLAS/graphblas-api-c/files/7541426/GraphBLAS_API_C_2.0.0.pdf).
+## Legacy versions
 
+Version 1.0 (provisional) of the C language API was released on May
+29, 2017 at the GABB workshop
+[here](http://graphanalysis.org/workshop2017.html).
+Version 1.1.0 (provisional) released on November 14, 2017.
+[Version 1.2.0](/docs/GraphBLAS_API_C_v1.2.0.pdf) was released on May 18, 2018.
 
 # Projects developing implementations of the GraphBLAS