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/L1SIM.md

@@ -25,14 +25,14 @@ multiple UEs.
 
 [[_TOC_]]
 
-# Build
+## Build
 
 Build eNB/UE as normal, as also described in [How to build the eNB and the UE](./BUILD.md):
 ```bash
 ./build_oai -c --ninja --eNB --UE
 ```
 
-# How to run an eNB with the noS1 option
+## How to run an eNB with the noS1 option
 
 The following paragraph(s) explains how to run the L1 simulator in noS1 mode and using the oai kernel modules.
 
@@ -48,7 +48,7 @@ $ cd ../ran_build/build
 $ sudo -E ./lte-softmodem -O YYY.conf --noS1
 ```
 
-# How to run a UE with the noS1 option
+## How to run a UE with the noS1 option
 
 Similarly modify the example configuration file in `/openairinterface5g/ci-scripts/conf_files/rru.band7.nos1.simulator.conf` and replace loopback interface and IP addresses. Copy your modifications to a new file, let's call XXX.conf the resulting configuration file.
 
@@ -64,7 +64,7 @@ $ sudo ./lte-uesoftmodem -O XXX.conf -r 25 --siml1 --noS1
 
 That should give you equivalent functionality to what you had with oaisim including noise and RF channel emulation (path loss / fading, etc.). You should also be able to run multiple UEs. 
 
-# How to ping an eNB from a UE and vice versa (with the noS1 option)
+## How to ping an eNB from a UE and vice versa (with the noS1 option)
 
 Once your eNB and UE (built with the noS1 option) are running and synchronised, you can ping the eNB from the UE with the following command: