Documentation: add missing platforms to platforms/ and remove introduction/platforms

Documentation/platforms/risc-v/c906/index.rst

@@ -0,0 +1,3 @@
+==========
+THEAD C906
+==========

Documentation/platforms/risc-v/hpm6750/index.rst

@@ -0,0 +1,3 @@
+===============
+Hpmicro HPM6750
+===============

Documentation/platforms/risc-v/k210/index.rst

@@ -0,0 +1,3 @@
+=============
+Kendryte K210
+=============

Documentation/platforms/risc-v/litex/index.rst

@@ -1,6 +1,6 @@
-======================================
+==========================
 Enjoy Digital LiteX FPGA's
-======================================
+==========================
 
 The LiteX framework provides a convenient and efficient infrastructure to create FPGA Cores/SoCs, to explore various digital design architectures and create full FPGA based systems. 
 
@@ -16,7 +16,7 @@ should work with either core, requiring minimal adjustment to the configuration.
 
 
 Toolchain
-==============
+=========
 
 Litex projects can be built with a generic RISC-V GCC toolchain. There are currently two options.
 

Documentation/platforms/risc-v/qemu/index.rst

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+======================
+QEMU Generic RV32/RV64
+======================

Documentation/platforms/risc-v/rv32m1/index.rst

@@ -0,0 +1,3 @@
+==========
+NXP RV32M1
+==========