chore: add release conventions document and refactor CI to run node release separately

[Daanvdplas]

This PR does two things (apologies for not separating them, will do next time).

1. Lines out the release conventions. View here: https://github.com/r0gue-io/pop-node/blob/daan/docs-release_conventions/docs/release.md
2. Refactors the CI so that it is clear how to manually run the job to build the release for a node without a runtime build (works). As well as having the CI pickup automatically when a release title is created with node (works).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.41%. Comparing base (8fb843f) to head (f6d78df).
Report is 2 commits behind head on main.

@@            Coverage Diff             @@
##             main     #433      +/-   ##
==========================================
+ Coverage   68.31%   71.41%   +3.10%     
==========================================
  Files          70       72       +2     
  Lines       11764    13568    +1804     
  Branches    11764    13568    +1804     
==========================================
+ Hits         8036     9690    +1654     
- Misses       3470     3605     +135     
- Partials      258      273      +15     

see 20 files with indirect coverage changes