Revise documentation for Imperial CX3 HPC Configuration

[davidhemp]

Updated documentation for the Imperial CX3 HPC configuration, including changes to installation instructions and account requirements.

---

name: Update config
about: Updating the docs and config for using on Imperial clusters

Please follow these steps before submitting your PR:

• If your PR is a work in progress, include [WIP] in its title
• Your PR targets the master branch
• You've included links to relevant issues, if any

Steps for adding a new config profile:

• Add your custom config file to the conf/ directory
• Add your documentation file to the docs/ directory
• Add your custom profile to the nfcore_custom.config file in the top-level directory
• Add your profile name to the profile: scope in .github/workflows/main.yml
• OPTIONAL: Add your custom profile path and GitHub user name to .github/CODEOWNERS (**/<custom-profile>** @<github-username>)

[davidhemp]

I've set -B /rds/,${TMPDIR}:/tmp,${TMPDIR}:/var/tmp as TMPDIR is set to the job specific storage on the compute node. This is automatically cleared at the end of the job.

[davidhemp]

@nfancy Would you be able to review these changes as well?