doc: Proxy settings should use systemd override

[jsirianni]

Description of Changes

When enabling proxy, we should be using a systemd override instead of editing the main service file. This is because the package manager (rpm / deb) will replace the systemd service. Using an override allows the user to make changes without preventing us from making changes to the main service.

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• CI passes

[codecov]

Codecov Report

Merging #595 (8ab72fe) into master (3b23e5c) will decrease coverage by 1.76%.
The diff coverage is 88.89%.

@@            Coverage Diff             @@
##           master     #595      +/-   ##
==========================================
- Coverage   76.69%   74.93%   -1.76%     
==========================================
  Files         132      133       +1     
  Lines        8308    10086    +1778     
==========================================
+ Hits         6371     7557    +1186     
- Misses       1473     2058     +585     
- Partials      464      471       +7     

Impacted Files	Coverage Δ
operator/buffer/buffer.go	75.86% <ø> (-1.92%)	⬇️
operator/buffer/memory.go	71.60% <ø> (-3.74%)	⬇️
...builtin/input/aws/cloudwatch/cloudwatch_persist.go	75.00% <ø> (-6.82%)	⬇️
operator/builtin/input/azure/event_hub.go	0.00% <0.00%> (ø)
operator/builtin/input/azure/event_hub_config.go	90.91% <ø> (-1.09%)	⬇️
operator/builtin/input/file/finder.go	100.00% <ø> (ø)
operator/builtin/input/file/reader.go	53.05% <0.00%> (-5.73%)	⬇️
operator/builtin/input/http/config.go	97.56% <ø> (-0.29%)	⬇️
operator/builtin/input/journald/journald.go	58.76% <ø> (-1.83%)	⬇️
operator/builtin/input/udp/udp.go	73.49% <ø> (-1.51%)	⬇️
... and 159 more

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