docs: ref-manual: ref-system-requirements: update requirements to build Sphinx docs

Tested with containers on all supported distributions.

Debian 8 (Jessie) still has Python3.4 and an old pip3, which makes it
impossible to build typing module which is a requirement of "new" Sphinx
python module.

One cannot update to latest pip3 from within pip in Jessie's version.
One cannot get a newer pip from upstream because newer pip don't support
Python3.4 anymore.
One cannot build with python3-sphinx package from Jessie because the
package is too old (1.2.3) and does not have sphinx.ext.autosectionlabel
module which appeared in 1.4 version.

(From yocto-docs rev: 14da565986a573ac7e0b5c5943e55b7b74f99dd5)

Signed-off-by: Quentin Schulz <foss@0leil.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/poky.yaml

@@ -86,3 +86,4 @@ CENTOS8_HOST_PACKAGES_ESSENTIAL : "-y epel-release
      socat perl-Data-Dumper perl-Text-ParseWords perl-Thread-Queue python3-pip \
      python3-GitPython python3-jinja2 python3-pexpect xz which SDL-devel xterm \
      rpcgen mesa-libGL-devel"
+PIP3_HOST_PACKAGES_DOC : "$ sudo pip3 install sphinx sphinx_rtd_theme pyyaml"

documentation/ref-manual/ref-system-requirements.rst

@@ -141,7 +141,14 @@ supported Ubuntu or Debian Linux distribution:
    Yocto Project documentation manuals:
    ::
 
-      $ sudo apt-get install make xsltproc docbook-utils fop dblatex xmlto
+      $ sudo apt-get install make python3-pip
+      &PIP3_HOST_PACKAGES_DOC;
+
+   .. note::
+
+      It is currently not possible to build out documentation from Debian 8
+      (Jessie) because of outdated ``pip3`` and ``python3``. ``python3-sphinx``
+      is too outdated.
 
 Fedora Packages
 ---------------
@@ -159,8 +166,8 @@ supported Fedora Linux distribution:
    Yocto Project documentation manuals:
    ::
 
-      $ sudo dnf install  docbook-style-dsssl docbook-style-xsl \
-      docbook-dtds docbook-utils fop libxslt dblatex xmlto
+      $ sudo dnf install make python3-pip which
+      &PIP3_HOST_PACKAGES_DOC;
 
 openSUSE Packages
 -----------------
@@ -178,7 +185,8 @@ supported openSUSE Linux distribution:
    Yocto Project documentation manuals:
    ::
 
-      $ sudo zypper install dblatex xmlto
+      $ sudo zypper install make python3-pip which
+      &PIP3_HOST_PACKAGES_DOC;
 
 
 CentOS-7 Packages
@@ -207,8 +215,8 @@ supported CentOS-7 Linux distribution:
    Yocto Project documentation manuals:
    ::
 
-      $ sudo yum install docbook-style-dsssl docbook-style-xsl \
-      docbook-dtds docbook-utils fop libxslt dblatex xmlto
+      $ sudo yum install make python3-pip which
+      &PIP3_HOST_PACKAGES_DOC;
 
 CentOS-8 Packages
 -----------------
@@ -239,8 +247,8 @@ supported CentOS-8 Linux distribution:
    Yocto Project documentation manuals:
    ::
 
-      $ sudo dnf install docbook-style-dsssl docbook-style-xsl \
-      docbook-dtds docbook-utils fop libxslt dblatex xmlto
+      $ sudo dnf install make python3-pip which
+      &PIP3_HOST_PACKAGES_DOC;
 
 Required Git, tar, Python and gcc Versions
 ==========================================