doc: fix files mis-named as PNG files

There are a few non-PNG (JPEG and WebP) files that are being
name as PNG files. This causes pdflatex/latexmk to fail due
to them not actually being PNG files. So rename those files
with correct extensions and update the RST files.

Also converted the WebP file in JPEG as PDFLatex cannot parse
WebP image.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

boards/arm/efm32pg_stk3402a/doc/index.rst

@@ -10,7 +10,7 @@ The EFM32 Pearl Gecko Starter Kit EFM32PG-STK3402A contains an MCU from the
 EFM32PG family built on an ARM® Cortex®-M4F processor with excellent low
 power capabilities.
 
-.. figure:: ./efm32pg_stk3402a.png
+.. figure:: ./efm32pg_stk3402a.jpg
    :width: 375px
    :align: center
    :alt: EFM32PG-SLSTK3402A

boards/arm/efr32_slwstk6061a/doc/index.rst

@@ -10,7 +10,7 @@ The EFR32 Flex Gecko Wireless Starter Kit SLWSTK6061A contains a Wireless
 System-On-Chip from the EFR32FG family built on an ARM® Cortex®-M4F processor
 with excellent low power capabilities.
 
-.. figure:: ./efr32_slwstk6061a.png
+.. figure:: ./efr32_slwstk6061a.jpg
    :width: 375px
    :align: center
    :alt: EFR32-SLWSTK6061A

boards/arm/lpcxpresso55s69/doc/index.rst

@@ -12,7 +12,7 @@ architecture. The board includes a high performance onboard debug probe, audio
 subsystem, and accelerometer, with several options for adding off-the-shelf
 add-on boards for networking, sensors, displays, and other interfaces.
 
-.. image:: ./lpcxpresso55s69.png
+.. image:: ./lpcxpresso55s69.jpg
    :width: 720px
    :align: center
    :alt: LPCXPRESSO55S69

boards/arm/olimex_stm32_h407/doc/index.rst

@@ -9,7 +9,7 @@ Overview
 The OLIMEX-STM32-H407 is open source hardware and is based on
 the STMicroelectronics STM32F407ZG ARM Cortex-M4 CPU.
 
-.. figure:: img/olimex_stm32_h407.png
+.. figure:: img/olimex_stm32_h407.jpg
      :width: 540px
      :align: center
      :height: 454px

doc/development_process/release_process.rst

@@ -187,7 +187,7 @@ Milestones
 The following graphic shows the timeline of phases and milestones associated
 with each release:
 
-.. figure:: milestones.png
+.. figure:: milestones.jpg
     :align: center
     :alt: Release Milestones
     :figclass: align-center
@@ -199,7 +199,7 @@ This shows how the phases and milestones of one release overlap with those of
 the next release:
 
 
-.. figure:: milestones2.png
+.. figure:: milestones2.jpg
     :align: center
     :alt: Release Milestones
     :figclass: align-center