chore: update installation.md for ARM user

[namkyu1999]

What this PR does / why we need it: I updated installation.md for ARM users. Briefly, the official MongoDB Helm chart does not support ARM, so I added commands for users to install Litmus with an unofficial MongoDB image.

Special notes for your reviewer: @Saranya-jena

Checklist:

• [ ] Fixes #
• [x] Signed the commit for DCO to be passed
• [x] Labelled this PR & related issue with documentation tag

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[namkyu1999]

Updated it @gdsoumya, please take a look!