docs: two-path CLI install (dbt v2 vs dbt Core 1.x with dbt1-* extras)

[haritamar]

Summary

Documents the new CLI install model introduced in #2360 (the CLI no longer has a hard dbt-core dependency):

• snippets/install-cli.mdx (shared by oss/cli-install, quickstart, tutorial, and report-UI pages) now splits installation into two paths:
  • dbt v2 (Fusion): pip install elementary-data with no extra, then install dbt/dbt-oss per dbt's installation guide — adapters are built in.
  • dbt Core 1.x: pip install 'elementary-data[dbt1-<warehouse>]', with a note that the legacy un-prefixed extras remain supported.
• Removed the stale elementary-data[vertica] line (no such extra exists in pyproject) and replaced it with pip install elementary-data dbt-vertica.
• oss/general/troubleshooting.mdx: updated the clean-venv reinstall example to use dbt1-<adapter> and marked the extra as dbt Core 1.x-only.

Base branch is docs per the docs workflow. Should be merged together with (or after) #2360, which adds the dbt1-* extras.

Link to Devin session: https://app.devin.ai/sessions/496f2bf70ae14ec1a1bccb245fbc9328
Open in Devin Desktop: https://app.devin.ai/desktop/session/496f2bf70ae14ec1a1bccb245fbc9328?variant=devin
Requested by: @haritamar

[devin-ai-integration]

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[jefersonmsantos]

Light docs review from #2354 / #2360 context:

• The split between dbt v2 (Fusion) — plain pip install elementary-data + separate dbt install — and dbt Core 1.x — dbt1-<warehouse> extras — matches the packaging in Remove the CLI's hard dbt-core dependency (dbt v2 ships as dbt/dbt-oss) #2360.
• Calling out that legacy extras ([snowflake], etc.) still work is helpful for existing users.
• Troubleshooting change (dbt1-<adapter> with a note that it's not needed for Fusion) is consistent.

One nit: in install-cli.mdx, linking to dbt's install guide is the right default; optional mention of DBT_FUSION_PATH / standalone binary installer (as in #2360's PR description) could help users who don't use pip for dbt v2 — not required for merge.

Looks good to ship alongside #2360.