Contributing guidelines should be consolidated in one place

[chalin]

• From a few threads of discussion on Slack, e.g., starting at https://cloud-native.slack.com/archives/C02UN96HZH6/p1718084686643089: I agree with @zeitlinger that our current multiple sources of contributing info is confusing. IMHO, there should be a single place where contributors are guided to, and that should be https://opentelemetry.io/docs/contributing/
• Also, https://opentelemetry.io/docs/contributing/ should not contain details. As a landing page, it should help contributors figure out which other pages to consult. The main "contributing new content" should be reworked and moved to a section subpage IMHO