docs(whatsnew): document show_whats_new config and env var

Summary

The whatsnew banner introduced in !3335 (merged) can already be disabled via the show_whats_new config key or the GLAB_SHOW_WHATS_NEW env var, but neither was surfaced in the two doc pages a user would actually look at to find an opt-out:

• https://docs.gitlab.com/cli/#environment-variables — generated from the help:environment annotation in internal/commands/root.go.
• https://docs.gitlab.com/cli/config/#synopsis — generated from the Long description in internal/commands/config/config.go.

Reported in #8374 by a user who was trying to suppress the banner in scripted glab repo clone output.

What's in here

• Add GLAB_SHOW_WHATS_NEW to the help:environment annotation in internal/commands/root.go.
• Add show_whats_new to the Current respected settings bullet list in internal/commands/config/config.go.
• Regenerate docs/source/_index.md and docs/source/config/_index.md via make gen-docs.

Follow-up

There is a broader gap here: internal/config/schema.go is the declared source of truth for every config key and env var glab knows about, but the two cobra strings above are hand-maintained and have drifted from it (several keys/env vars in the schema are not documented in either place). A follow-up issue will track refactoring those strings to be generated from KeySchema so this kind of miss can't happen again.

Test plan

• glab help shows GLAB_SHOW_WHATS_NEW in the ENVIRONMENT VARIABLES block.
• glab config --help shows the show_whats_new bullet.
• docs/source/_index.md and docs/source/config/_index.md regenerate cleanly under make gen-docs (no further diff).