docs: add PR templates for the new release process

[luislhl]

Acceptance Criteria

• We should have multiple PR templates for the different kinds of PRs, as agreed in https://github.com/HathorNetwork/internal-issues/issues/151 (search for "PR templates")
• The solution is based on https://stackoverflow.com/a/75030350/3477266

[codecov]

Codecov Report

Merging #674 (640f4b2) into master (21443ac) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #674   +/-   ##
=======================================
  Coverage   84.10%   84.11%           
=======================================
  Files         246      246           
  Lines       20442    20442           
  Branches     2757     2757           
=======================================
+ Hits        17192    17194    +2     
  Misses       2674     2674           
+ Partials      576      574    -2     

see 2 files with indirect coverage changes