Add docs on milestone percentage stats

Clarify how the milestone percentage is calculated in the docs.
Include example image of milestone.
47 jobs for mrchrisw-add-mr-stats in 59 minutes and 34 seconds (queued for 38 seconds)
Status Job ID Name Coverage
  Build
manual #17138981
allowed to fail manual
build-package
 
  Prepare
passed #17138982
gitlab-org
knapsack

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setup-test-env

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  Test
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docs lint

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downtime_check

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ee_compat_check

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rake gitlab:assets:compile

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rake karma

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48.76%
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rake mysql db:migrate:reset

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rake mysql db:rollback

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rake mysql db:seed_fu

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rake pg db:migrate:reset

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rake pg db:rollback

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rake pg db:seed_fu

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rspec pg 0 20

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rspec pg 12 20

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rspec pg 16 20

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rspec pg 19 20

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rspec pg 2 20

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rspec pg 3 20

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rspec pg 4 20

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rspec pg 5 20

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rspec pg 6 20

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rspec pg 7 20

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rspec pg 8 20

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rspec pg 9 20

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spinach pg 0 10

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spinach pg 4 10

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spinach pg 5 10

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spinach pg 6 10

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spinach pg 7 10

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spinach pg 8 10

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spinach pg 9 10

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static-analysis

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  Post Test
skipped #17139043
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coverage
skipped #17139045
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lint:javascript:report
 
Name Stage Failure
failed
rspec pg 3 20 Test There is an unknown failure, please try again
rspec ./spec/features/merge_requests/assign_issues_spec.rb:35 # Merge request issue assignment logged in as author doesn't display if related issues are already assigned
rspec ./spec/features/merge_requests/assign_issues_spec.rb:45 # Merge request issue assignment not MR author doesn't not show assignment link

Uploading artifacts...
coverage/: found 5 matching files

knapsack/: found 7 matching files

tmp/capybara/: found 57 matching files

Uploading artifacts to coordinator... ok
id=17138993 responseStatus=201 Created token=LAuN3V33
ERROR: Job failed: exit code 1