overview-manual: convert source-fetching.png to svg and fix UNPACKDIR

- Convert the png file to an SVG file.
- Add the new UNPACKDIR directory to the image.
- Remove ${PR} from WORKDIR value, after cc83e4548465 ("bitbake.conf:
  Drop PE and PR from WORKDIR and STAMP") on OE-Core.
- Change S value to BP (equal to ${BPN}-${PV}, but more accurate).-

This fixes [YOCTO #15730].

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: 404a0fb167402e13d3a4ce5aba23aa22a78a0c06)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 27725e4e7bf4d5fe7ad222de077cc693b9205b17)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/overview-manual/concepts.rst

@@ -683,7 +683,7 @@ Source Fetching
 The first stages of building a recipe are to fetch and unpack the source
 code:
 
-.. image:: figures/source-fetching.png
+.. image:: svg/source-fetching.*
    :width: 100%
 
 The :ref:`ref-tasks-fetch` and :ref:`ref-tasks-unpack` tasks fetch