docs(.github): put PR template instructions in comments

[michaelbeaumont]

Checklist prior to review

• Link to docs PR or issue --
• Link to UI issue or PR --
• Is the issue worked on linked? --
• Unit Tests --
• E2E Tests --
• Manual Universal Tests --
• Manual Kubernetes Tests --
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? --

[codecov-commenter]

Codecov Report

Merging #4680 (d031943) into master (93056a7) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4680   +/-   ##
=======================================
  Coverage   46.47%   46.48%           
=======================================
  Files         684      684           
  Lines       46500    46500           
=======================================
+ Hits        21613    21617    +4     
+ Misses      22976    22973    -3     
+ Partials     1911     1910    -1     

Impacted Files	Coverage Δ
pkg/xds/generator/direct_access_proxy_generator.go	90.90% <0.00%> (+1.13%)	⬆️
pkg/xds/cache/once/once.go	100.00% <0.00%> (+5.00%)	⬆️
pkg/xds/cache/once/cache.go	87.17% <0.00%> (+5.12%)	⬆️

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