docs(site): Add guideline for localization

[lupusA]

Hi,

this PR adds the ability to support localisation in kopia-UI. The PR contains the following changes:

1. Adds a guidline to outline the localisation process

This PR has a dependency to kopia/htmlui#241

Feedback is welcomed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.96%. Comparing base (cb455c6) to head (37dffde).
Report is 92 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3776      +/-   ##
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+ Coverage   75.86%   76.96%   +1.09%     
==========================================
  Files         470      476       +6     
  Lines       37301    29039    -8262     
==========================================
- Hits        28299    22349    -5950     
+ Misses       7071     4749    -2322     
- Partials     1931     1941      +10     

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[julio-lopez]

Suggested change

	## Localisation Guidlines
	## Localisation Guidelines

[julio-lopez]

Suggested change

```kopiaUI``` features different tests to prevent situations where translations become obsolet and or unused. The tests check for keys with empty or null values and that keys exist in all provided translation files. The tests will fail if translation files contain empty keys or if keys are not declared in all files. Additionally, the test extracts the currently used translation keys within the code to check if all keys are used at least once.

```kopiaUI``` features different tests to prevent situations where translations become obsolete and or unused. The tests check for keys with empty or null values and that keys exist in all provided translation files. The tests will fail if translation files contain empty keys or if keys are not declared in all files. Additionally, the test extracts the currently used translation keys within the code to check if all keys are used at least once.