lake: add UDF docs

[awxxxxxx]

What is changed, added or deleted? (Required)

This PR publishes TiDB Cloud Lake UDF (user-defined functions) documentation with minimal changes to existing pages:

• adds a comprehensive guide for choosing among SQL, Python, JavaScript, WebAssembly, aggregate, table, and external UDFs;
• exposes the External AI Functions guide and External Function CREATE FUNCTION reference in the Lake TOC;
• preserves the existing External AI and command-page structure while using tidbcloudlake-udf and tidbcloudlake_udf in external Python examples;
• updates external examples to use a public HTTPS endpoint and documents the tenant UDF server allowlist requirement;
• fixes External Function management links.

The examples were validated against the published tidbcloudlake-udf v0.2.20 package and a staging Lake warehouse. SQL scalar/table UDFs and Python/JavaScript aggregate UDFs passed. External registration correctly returned Unallowed UDF server address for a hostname not yet added to the tenant allowlist.

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

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

• This PR is translated from:
• Other reference link(s):
  • https://github.com/tidbcloud/lake-udf
  • https://pypi.org/project/tidbcloudlake-udf/

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[Copilot]

Pull request overview

This PR adds and surfaces TiDB Cloud Lake UDF documentation, including a new guide to help users choose among UDF types, and updates the External Function docs and examples to use the tidbcloudlake-udf package and public HTTPS endpoints.

Changes:

• Adds a new “Choose a User-Defined Function Type” guide and links it from the Lake TOC.
• Exposes the “External AI Functions” guide in the Lake TOC and updates Python examples to use tidbcloudlake-udf.
• Updates External Function navigation and examples, including adding the CREATE FUNCTION reference entry.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
TOC-tidb-cloud-lake.md	Adds TOC entries for External AI Functions, the UDF type selection guide, and External Function command references.
tidb-cloud-lake/sql/external-function.md	Updates the External Function overview table to point at the correct command pages (and should align link text with actual SQL statements).
tidb-cloud-lake/sql/create-function.md	Updates the external function end-to-end example to use tidbcloudlake-udf and a public HTTPS ADDRESS.
tidb-cloud-lake/sql/alter-function-sql.md	Updates external function ADDRESS examples to a public HTTPS endpoint.
tidb-cloud-lake/guides/external-ai-functions.md	Updates the external AI function example to use tidbcloudlake-udf.
tidb-cloud-lake/guides/choose-a-udf-type.md	Adds a new guide explaining how to choose between SQL/script/WASM/aggregate/table/external UDF types and operational considerations.

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