[editorial] Move supplementary-guidelines into docs

[chalin]

• Fixes [editorial] Move supplementary-guidelines to be under docs #1240
• Moves supplementary-guidelines into docs and renames it to non-normative
• Renames semantic_conventions_code_generation.md to libraries.md and adjust the linkTitle - PTAL
• Fixes links into the supplementary-guideline pages
• Closes Move http migration guide to supplementary-guidelines directory #1234 by superseding it
  • Moves & renamed the file: docs/http/migration-guide.md → docs/non-normative/http-migration.md
  • Adds a Hugo redirect alias
• Normalizes a few other page aliases by dropping unnecessary path prefixes

Screenshot

This is what it will look like on the OTel website once integrated:

[chalin]

/cc @open-telemetry/docs-maintainers

[joaopgrassi]

@chalin did you also intend to move the HTTP migration guide document as part of this PR? I don't see it in the changes 🤔 #1234

[chalin]

@chalin did you also intend to move the HTTP migration guide document as part of this PR? I don't see it in the changes 🤔 #1234

LOL ... the whole reason for this change! For some reason, in the back of my mind, that page was already a part of the supp. guidelines 🤷🏼‍♂️. I'm moving the HTTP migration page now.

[chalin]

Ready for a final review @joaopgrassi et al.