[PATCH 2/7] doc: make pdfdocs: Update the description with the needed steps

[Adriano Carvalho]

It is not clear that before "make pdfdocs" it is also necessary to install "doc/sphinx/requirements.txt".
If I don't install "doc/sphinx/requirements.txt", this is what I get with docker.io/trini/u-boot-gitlab-ci-runner:jammy-20250714-25Jul2025:

uboot at 35c89b9ab70e:~/u-boot$ make pdfdocs
  SPHINX  latexdocs --> file:///home/uboot/u-boot/doc/output/latex
  PARSE   include/linker_lists.h
Running Sphinx v4.3.2
Warning: The Sphinx 'sphinx_rtd_theme' HTML theme was not found. Make sure you have the theme installed to produce pretty HTML output. Falling back to the default theme.
loading translations [en]... done

Extension error:
Could not import extension sphinx-prompt (exception: No module named 'sphinx-prompt')
make[1]: *** [doc/Makefile:84: latexdocs] Error 2
make: *** [Makefile:2686: pdfdocs] Error 2

A "t" was also missing in "documenation". This commit fixes that also.
NOTE: This won't completely fix the pdfdocs target. See next commit(s).

Signed-off-by: Adriano Carvalho <adrianocarvalho.pt at gmail.com>
---
 doc/build/documentation.rst | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/doc/build/documentation.rst b/doc/build/documentation.rst
index 68d7e11c572..2540f3973d2 100644
--- a/doc/build/documentation.rst
+++ b/doc/build/documentation.rst
@@ -65,8 +65,22 @@ The *infodocs* target builds both a texinfo and an info file:
 PDF documentation
 -----------------
 
-The *pdfdocs* target is meant to be used to build PDF documenation.
-As v2023.01 it fails with 'LaTeX Error: Too deeply nested'.
+The *pdfdocs* target is meant to be used to build PDF documentation:
+
+.. code-block:: bash
+
+    # Create Python environment 'myenv'
+    python3 -m venv myenv
+    # Activate the Python environment
+    . myenv/bin/activate
+    # Install build requirements
+    python3 -m pip install -r doc/sphinx/requirements.txt
+    # Build the documentation
+    make pdfdocs
+    # Deactivate the Python environment
+    deactivate
+
+However, as of v2023.01 it fails with 'LaTeX Error: Too deeply nested'.
 
 We can use texi2pdf instead:
 
-- 
2.48.1