doc: improve latex look and feel

This patch improves the general look and feel of the LaTeX (PDF)
documentation build. Changes include:

- A custom title with relevant information has been created
- Some colors have been adjusted to match those in the web template
- Charter font family is used

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

doc/_static/latex/preamble.tex

@@ -0,0 +1,15 @@
+% LaTeX documentation preamble
+%
+% Copyright (c) 2021 Nordic Semiconductor ASA
+% SPDX-License-Identifier: Apache-2.0
+
+\usepackage[some]{background}
+\usepackage{sectsty}
+
+\definecolor{bg-color}{HTML}{333f67}
+
+\setcounter{tocdepth}{2}
+
+\addto\captionsenglish{\renewcommand{\contentsname}{Table of contents}}
+
+\allsectionsfont{\color{bg-color}}

doc/_static/latex/title.tex

@@ -0,0 +1,44 @@
+% LaTeX documentation title page
+%
+% Copyright (c) 2021 Nordic Semiconductor ASA
+% SPDX-License-Identifier: Apache-2.0
+
+\makeatletter
+\newgeometry{top=3cm,left=0cm,right=0cm,bottom=2.5cm}
+
+\backgroundsetup{
+    scale=2.3,
+    contents={\sphinxlogo},
+    opacity=0.2,
+    angle=0,
+    position={0.25\textwidth,-0.4\textheight}
+}
+
+\BgThispage
+
+\begin{minipage}{2cm}
+    \color{bg-color} \rule{2cm}{1.7cm}
+\end{minipage}
+\hspace{0.2cm}
+\begin{minipage}{3cm}
+    \sphinxlogo
+\end{minipage}
+\hspace{0.2cm}
+\begin{minipage}{15cm}
+    \Huge \textbf{\@title}\\
+    \LARGE \py@release\releaseinfo
+\end{minipage}
+
+\vspace{21cm}
+
+\begin{flushright}
+    \begin{minipage}{7cm}
+        \large \@author\\\@date
+    \end{minipage}
+    \begin{minipage}{1.5cm}
+        \color{bg-color} \rule{1.5cm}{1.3cm}
+    \end{minipage}
+\end{flushright}
+
+\restoregeometry
+\makeatother