docs: update env vars table to use GLAB_ prefixed names

Description

Follow-up to !2347 (merged), which deprecated the non-prefixed environment variables (DEBUG, FORCE_HYPERLINKS, NO_PROMPT, GLAMOUR_STYLE) in favour of GLAB_-prefixed equivalents.

The docs/source/_index.md environment variables table was not updated as part of that MR. This MR:

• Replaces the deprecated variable names with their canonical GLAB_-prefixed versions as the primary entry in the table.
• Adds a note in each affected row's description mentioning the deprecated name (for example: Deprecated: GLAMOUR_STYLE).

Affected rows:

• DEBUG → GLAB_DEBUG
• FORCE_HYPERLINKS → GLAB_FORCE_HYPERLINKS
• NO_PROMPT → GLAB_NO_PROMPT
• GLAMOUR_STYLE → GLAB_GLAMOUR_STYLE

Related Issues

Relates to #7965 (closed)