Improves Proxmox Custom Service

[luixal]

Description

As users started using the Proxmox custom service, some improvements came up and are included in this PR:

• Avoid showing zeroed values: while loading, default values (0) where shown. No a loading message is shown.
• Iconography: icons added to show states (loading and error) on the indicator slot.
• Documentation: some info about permissions needed in proxmox. Also added a comment on possible CORS problem.

Fixes #520

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I've read & comply with the contributing guidelines
• I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• I have made corresponding changes to the documentation (README.md).
• I've checked my modifications for any breaking changes, especially in the config.yml file

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[tobidemski]

lgtm expect the little typo