tidb-lightning: update docs (#2946)

[ti-srebot]

cherry-pick #2946 to release-4.0

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What is changed, added or deleted? (Required)

Incorporate changes for these new features since last time:

• TLS config
• TiKV Importer now supports listening on 0.0.0.0
• TiKV Importer no longer requires Pushgateway for Prometheus
• CSV strict-format config for optional parallel speed up
• TiDB's default SQL mode is no longer strict
• Several new CLI parameters are exposed

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from:

• Other reference link(s):

  This PR involves changes introduced from:

  • config: synchronize actual default value with the toml file tidb-lightning#255
  • Support TLS; Reduce the need of config.toml in integration tests tidb-lightning#270
  • lightning: split large csv file if possible tidb-lightning#272
  • Some SwitchMode improvements tidb-lightning#287
  • config: remove strict mode from default SQL mode tidb-lightning#316
  • *: support TLS tikv/importer#40
  • setup status server for profiling tikv/importer#46

[ti-srebot]

@kennytm please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/docs/invitations

[yikeke]

LGTM