Add RFC for Document-Level Alerting

[qreshi]

Issue #, if available:

Description of changes:
Add RFC for Document-Level Alerting feature

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #327 (9a8bbc6) into master (8daf9e6) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #327      +/-   ##
============================================
- Coverage     79.33%   79.30%   -0.04%     
  Complexity      202      202              
============================================
  Files           151      151              
  Lines          5324     5324              
  Branches        701      701              
============================================
- Hits           4224     4222       -2     
- Misses          725      728       +3     
+ Partials        375      374       -1     

Impacted Files	Coverage Δ	Complexity Δ
...roforelasticsearch/alerting/alerts/AlertIndices.kt	65.64% <0.00%> (-1.53%)	0.00% <0.00%> (ø%)

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[praveensameneni]

https://discuss.opendistrocommunity.dev/t/trigger-for-more-than-one-server/2062