docs(influxdb3): add influxdb3 import to Enterprise write-data pages

## Pages to update

- https://docs.influxdata.com/influxdb3/enterprise/write-data/
- https://docs.influxdata.com/influxdb3/enterprise/write-data/influxdb3-cli/
- https://docs.influxdata.com/influxdb3/enterprise/write-data/best-practices/

If these pages use shared content under \`content/shared/\`, gate the import
sections with \`{{% show-in "enterprise" %}}\` / \`{{% /show-in %}}\` so Core
is unaffected. Core has no corresponding bulk import path.

## What's needed

- **write-data/_index.md**: Add \`influxdb3 import\` to the list of write
methods as the bulk/historical data load path.
- **write-data/influxdb3-cli/**: Add a section covering
\`influxdb3 import upload\` and \`influxdb3 import list\` with a link to
the CLI reference at \`/influxdb3/enterprise/reference/cli/influxdb3/import/\`.
- **write-data/best-practices/**: Note when bulk import is preferable over
streaming writes (large historical datasets, migration, pre-existing Parquet
files). Mention the compaction delay before imported rows are queryable.

## Requirements

- \`influxdb3 import\` requires the storage engine upgrade (\`--use-pacha-tree\`)
- Target database and table must exist before importing
- Data becomes queryable after compaction, not immediately on upload

## Related

- New CLI reference: \`/influxdb3/enterprise/reference/cli/influxdb3/import/\`
- Admin guide: \`/influxdb3/enterprise/admin/import-data/\`
- Relates to: influxdata/docs-v2#7381

Start with write-data/_index.md, write-data/influxdb3-cli/, and write-data/best-practices/, then compare any shared content under content/shared/. Add the enterprise-gated import guidance, CLI reference link, prerequisites, and compaction behavior; done means all three Enterprise pages cover the requested import workflow without changing Core content.

Written by the indexing model from the issue text.