[editorial] Drop path_base_for_github_subdir from front matter

[chalin]

• Contributes to [editorial] Inline Hugo front matter #1821
• Drops path_base_for_github_subdir from the Hugo front matter for all docs/**/README.md files. Note that we keep the cascaded value in the docs root readme.

[lmolkova]

I have no idea what path_base_for_github_subdir does, but I trust you @chalin :)

[chalin]

I have no idea what path_base_for_github_subdir does, but I trust you @chalin :)

@lmolkova - every doc page on OTel.io has a "View source" link. path_base_for_github_subdir is used to compute the value of the "View source" URL for pages whose source isn't the OTel.io repo itself. README.md files were special in that they needed to be renamed, and so each such file required its own path_base config. We won't need that any more (to know why, see open-telemetry/opentelemetry.io#6103). I hope that helps a bit to understand the purpose of that field. :)

(Btw, the "View source" is temporarily broken. A fix should land soon: open-telemetry/opentelemetry.io#6107.)