Introduce central 00_Index.md map and normalize cross-section links

[bubbobne]

Motivation

• Create a compact, centralized map of the vault to make top-level navigation and cross-domain entry points explicit.
• Standardize index link targets so section landing pages consistently point to 00_Index.md files rather than folder roots.

Description

• Add a new top-level map file 00_Index.md that lists core hubs and cross-domain links.
• Update the main Readme.md and multiple section index files to reference 00_Index.md targets, including Hydrology/00_Index.md, GEOframe/00_Index.md, Programming/00_Index.md, and Linux/00_Index.md.
• Add a legacy EndeavourOS note EndeavourOS.md and expand the EndeavourOS subindex Linux/EndeavourOS/00_Index.md with additional entries (Java.md, NordVPN_EndeavourOS_Guida.md) and a backlink to the legacy note.
• Add small content and navigation adjustments across GEOframe/00_Index.md, Hydrology/00_Index.md, Programming/00_Index.md, and Linux/00_Index.md, including a new Java Refresh entry and an HPC useful commands note.

Testing

• No automated tests were configured or run for these documentation changes.

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Codex Task