[ZEPPELIN-6430] Clean up removed R interpreter docs

[jongyoul]

What changed

This cleans up documentation references left behind after the R interpreter, SparkR, and rlang code removal in ZEPPELIN-6430.

• Removed R build/runtime requirements from docs where they are no longer Zeppelin requirements.
• Removed %spark.r and R example blocks, and updated the generic interpreter example to use %spark.sql.
• Updated stale env_python_3_with_R.yml links to the current Python environment file.
• Removed R example links and support-list entries from interpreter, quickstart, Livy, VM, ZeppelinContext, and SDK docs.

Why

The code removal left docs that still implied R interpreter or SparkR support. These updates avoid sending users toward removed functionality while keeping supported Python, Spark, SQL, and Mahout docs intact.

Validation

• git diff --check
• rg -n "SparkR|R interpreter|r-base|r-cran|IRkernel|R Tutorial|%spark\\.r|%r\\b|env_python_3_with_R" docs --glob "*.md"
• Additional strict search for actual R feature remnants returned no matches; the broad search only leaves false positives such as interpreter-base / interpreter-based.

[Copilot]

Pull request overview

This PR updates Zeppelin’s documentation to remove or replace references to the removed R interpreter / SparkR functionality (per ZEPPELIN-6430), ensuring the docs only describe currently supported interpreters and environments.

Changes:

• Removes R-related example links, build/runtime requirements, and documentation sections that referenced %spark.r / R interpreter usage.
• Updates generic interpreter examples to use supported %spark.sql (and similar) instead of R.
• Fixes stale Docker Python environment links by pointing to the current env_python_3.yml.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/usage/zeppelin_sdk/session_api.md	Removes the R client example link from the SDK session API page.
docs/usage/other_features/zeppelin_context.md	Removes R-specific ZeppelinContext usage example and updates interpreter list wording.
docs/usage/interpreter/overview.md	Replaces the %spark.r interpreter example with %spark.sql.
docs/setup/deployment/virtual_machine.md	Removes R packages from the VM contents list.
docs/setup/basics/how_to_build.md	Removes R packages from build requirements/install steps.
docs/quickstart/install.md	Removes R from the “Spark, Flink, SQL, Python…” quickstart section header.
docs/interpreter/spark.md	Removes R mentions from Spark interpreter docs and updates Docker env link to env_python_3.yml.
docs/interpreter/python.md	Updates Docker env link to env_python_3.yml.
docs/interpreter/mahout.md	Removes the Mahout→R visualization section and adjusts the related heading.
docs/interpreter/livy.md	Removes R mentions from Livy interpreter capability and shared SparkContext description.

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[jongyoul]

@pan3793 Hello, I found some more places mentioning R. Could you please review it?

[jongyoul]

Merged into master (d9021ef).