Recommend upgrading and up aggregate on README.md

Recommend upgrading the pip tools to ensure the user is running the
latest version of Doctools and the other tools.
Update aggregate info, removing monolithic mention in favor of symbolic
build.

Signed-off-by: Jorge Marques <jorge.marques@analog.com>

README.md

@@ -9,19 +9,19 @@ Ensure pip is newer than version 23.
 ```
 pip install pip --upgrade
 ```
-Install the documentation tools.
+Install/upgrade the documentation tools.
 ```
-(cd docs ; pip install -r requirements.txt)
+(cd docs ; pip install -r requirements.txt --upgrade)
 ```
 Build the documentation with Sphinx.
 ```
 (cd docs ; make html)
 ```
 The generated documentation will be available at `docs/_build/html`.
 
-## Building the monolithic documentation
+## Building the all documentations
 
-The monolithic version aggregates all ADI's documentation into a single output/webpage.
+It's possible to build all ADI's documentation at once.
 To generate it, considering Doctools is installed, do:
 ```
 adoc aggregate -d docs_output