docs: Standardize documentation headings (remove multiple # H1)

- Remove multiple single top-level (# H1) headings per file
 - Use (# H2 and ###### H6) sub-headings for consistency
 - Remove any trailing # in the headings
 - This ensures proper table of contents rendering in MkDocs.

 - Also removed RUN_NR_multiple_BWPs.md as it is moved to MAC usage

doc/GET_SOURCES.md

@@ -2,7 +2,7 @@ The OpenAirInterface software can be obtained from our gitLab server. You will
 need a git client to get the sources. The repository is used for main
 developments.
 
-# Prerequisites
+## Prerequisites
 
 You need to install `git` using the following commands:
 
@@ -11,12 +11,12 @@ sudo apt-get update
 sudo apt-get install git
 ```
 
-# Clone the Git repository (for OAI Users without login to gitlab server)
+## Clone the Git repository (for OAI Users without login to gitlab server)
 
 The [openairinterface5g repository](https://gitlab.eurecom.fr/oai/openairinterface5g.git)
 holds the source code for the RAN (4G and 5G).
 
-## All users, anonymous access
+### All users, anonymous access
 
 Clone the RAN repository:
 
@@ -24,7 +24,7 @@ Clone the RAN repository:
 git clone https://gitlab.eurecom.fr/oai/openairinterface5g.git
 ```
 
-## For contributors
+### For contributors
 
 Configure git with your name/email address, important if you are developer and
 want to contribute by pushing code. Please put your full name and the e-mail
@@ -37,7 +37,7 @@ git config --global user.email "Your email address"
 
 More information can be found in [the contributing page](../CONTRIBUTING.md).
 
-# Which branch to checkout?
+## Which branch to checkout?
 
 - `develop`: contains recent commits that are tested on our CI test bench. The
   update frequency is about once a week. 5G is only in this branch. **It is the