'OpenMPI with UCX' doc missing git submodule flag

[TysonRayJones]

Describe the bug

The OpenMPI with UCX section of the doc instructs to clone OpenMPI via:

git clone https://github.com/open-mpi/ompi.git
cd ompi
./autogen.pl

However, invocation of ./autogen.pl will throw an error:

The submodule "config/oac" is missing

This is corrected by using the --recurse-submodules option to git, replacing the initial line above with:

git clone --recurse-submodules https://github.com/open-mpi/ompi.git

Steps to Reproduce

• Follow verbatim the current instructions on this page of the doc.
• UCX version used: 1.17.0.
• Output of ucx_info -v is:

  # Library version: 1.17.0
  # Library path: /lib/libucs.so.0
  # API headers version: 1.17.0
  # Git branch 'master', revision ab5fa33
  # Configured with: --disable-logging --disable-debug --disable-assertions --disable- params-check
  

- Git version: `2.25.1`

[TysonRayJones]

If you make a revision to that doc page, it may be useful to point at:

• all the step numbers are "1" as I highlight here in yellow:

  

• running OpenMPI's configure script required sudo. So the below line in Sec. 'Building'

   ../configure --prefix=<ompi-install-path> --with-ucx=<ucx-install-path>

  should be prefixed with sudo
• installing openmpi from source, as that doc page instructs, requires that flex is installed. It might be useful to point this out. So the first step of this section (before the user runs OpenMPI's configure script) could include step:

  sudo apt-get install flex

[TysonRayJones]

Just realised I can PR this myself to running.md, which I may do after getting the build working