Split helm chart into another repo

[JaredTan95]

What's the purpose of this PR

Split helm chart into another repo: https://github.com/apolloconfig/apollo-helm-chart
@nobodyiam We need to update the domain point to new helm chart? Or Merge this after new helm chart works.

Which issue(s) this PR fixes:

Fixes ##3959

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[codecov-commenter]

Codecov Report

Merging #4125 (54c3642) into master (75f9950) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4125      +/-   ##
============================================
- Coverage     52.62%   52.52%   -0.10%     
+ Complexity     2616     2609       -7     
============================================
  Files           484      484              
  Lines         15186    15186              
  Branches       1572     1572              
============================================
- Hits           7991     7977      -14     
- Misses         6641     6656      +15     
+ Partials        554      553       -1     

Impacted Files	Coverage Δ
...work/apollo/biz/message/DatabaseMessageSender.java	50.00% <0.00%> (-14.59%)	⬇️
...framework/apollo/openapi/entity/ConsumerAudit.java	42.42% <0.00%> (-6.07%)	⬇️
...rk/apollo/spring/property/SpringValueRegistry.java	83.78% <0.00%> (-5.41%)	⬇️
...nfigservice/service/AccessKeyServiceWithCache.java	82.65% <0.00%> (-2.05%)	⬇️
.../framework/apollo/spring/property/SpringValue.java	87.71% <0.00%> (-1.76%)	⬇️
...mework/apollo/openapi/service/ConsumerService.java	53.38% <0.00%> (-1.70%)	⬇️
.../apollo/internals/RemoteConfigLongPollService.java	78.31% <0.00%> (+1.20%)	⬆️

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[nobodyiam]

@JaredTan95

The url https://www.apolloconfig.com/charts currently points to the charts directory, I think we also need to copy this charts folder to apollo-helm-chart.
As for the domain for apollo-helm-chart, how about charts.apolloconfig.com?
BTW, to keep backward compatibility, I think the charts directory should remain for a while so that users who haven't updated the charts repo could still fetch the current versions.

[nobodyiam]

I have copied the charts directory to docs directory of apollo-helm-chart. And now the new helm repo url is https://charts.apolloconfig.com/.
So we could now merge this pr and I will submit another pr to update the chart repo URL later.

[nobodyiam]

LGTM