docs: adopt docs restructure from provider submodules

[jn-av]

Note

This is a draft PR. Submodule pointers will be updated after the following PRs are merged:

• docs: restructure docs into end-user-guides and contribution-notes crossplane-provider-btp#613
• docs: restructure docs into end-user-guides and contribution-notes crossplane-provider-hana#75

What this PR does

Adopts the docs restructure done in both provider repositories. All guides are now consistently organized into two top-level sections across every provider:

• docs/end-user-guides/ — all end-user relevant guides
• docs/contribution-notes/ — all contributor-relevant work (ADRs, dev decisions, setup info)

Summary of changes

• Updates submodule pointers for crossplane-provider-btp and crossplane-provider-hana to the restructured docs branches
• Fixes footer links in docusaurus.config.js to reflect new paths

TODO before merge

• docs: restructure docs into end-user-guides and contribution-notes crossplane-provider-btp#613 is merged
• docs: restructure docs into end-user-guides and contribution-notes crossplane-provider-hana#75 is merged
• Update submodule pointers to the new main commits of both providers