doc: Fix platform port inclusion

This patch:

- Adds any leftover platform ports that were not having their
  documentation built (not in the index.rst table of contents)
- Corrects a handful of RST formatting errors that cause poor
  rendering
- Reorders the list of platforms so that they are displayed
  in alphabetical order

Change-Id: If8c135a822d581c3c5c4fca2936d501ccfd2e94c
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index eaeee84..2695d4e 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -7,24 +7,28 @@
    :numbered:
 
    allwinner
+   meson-gxbb
+   meson-gxl
    fvp_ve
-   imx8
-   imx8m
+   hikey
+   hikey960
+   intel-agilex
    intel-stratix10
-   ls1043a
    marvell/index
-   meson-gxbb
-   meson-gxl
    mt8183
    nvidia-tegra
+   warp7
+   imx8
+   imx8m
+   ls1043a
+   poplar
    qemu
+   rpi3
    rcar-gen3
    rockchip
-   rpi3
    socionext-uniphier
-   stm32mp1
    synquacer
+   stm32mp1
    ti-k3
-   warp7
    xilinx-versal
    xilinx-zynqmp