Add documentation on installing ERT on ARM

[aronhoyer]

Issue
Resolves #5797

Approach
Added description of the required steps to get ERT running on an ARM CPU under the Installation section.

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.
• Updated documentation
• Ensured new behaviour is tested

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[dafeda]

Nice 👍

[codecov-commenter]

Codecov Report

Merging #5798 (ba5449b) into main (824f6ea) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #5798   +/-   ##
=======================================
  Coverage   82.61%   82.61%           
=======================================
  Files         352      352           
  Lines       21851    21851           
  Branches      826      826           
=======================================
  Hits        18052    18052           
  Misses       3500     3500           
  Partials      299      299           

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