ci: add -subdir flag to gen-docs to support cloud CLI extension docs

[lennessyy]

Summary

The Temporal Cloud CLI extension needs auto-generated command reference documentation, just like the core CLI commands. The extension has ~130 commands across 10 top-level groups (namespace, user, apikey, account, etc.), and the documentation repo's workflow already uses gen-docs with multiple -input flags to generate docs from YAML command definitions.

The problem is that gen-docs currently produces one file per top-level command. Since all cloud commands share the cloud root, they'd all land in a single cloud.mdx file (~2800 lines), which is too large to be useful.

This PR adds a -split flag that tells gen-docs to output a command's subcommands as separate files in a subdirectory. The documentation repo's workflow will pass -split cloud to produce individual pages:

cloud/namespace.mdx (1148 lines)
cloud/account.mdx (318 lines)
cloud/user-group.mdx (417 lines)
cloud/user.mdx (262 lines)
cloud/apikey.mdx (253 lines)
cloud/nexus.mdx (212 lines)
cloud/connectivity.mdx (144 lines)
cloud/login.mdx
cloud/logout.mdx
cloud/whoami.mdx

No changes are needed in the cloud-cli repo. The split is controlled entirely by the documentation workflow:

go run ./cmd/gen-docs \
-input internal/temporalcli/commands.yaml \
-input path/to/cloud-cli/commands.yml \
-split cloud \
-output dist/docs

Also fixes the encodeJSONExample regex to catch standalone JSON objects in single quotes (e.g., '{"key":"value"}'), which caused MDX rendering errors.

Test plan

• CLI-only generation produces identical flat output (no regressions)
• -split cloud produces cloud/ subdirectory with correct file splits
• Generated content (frontmatter, headings, flag tables, global flags) renders correctly
• JSON-in-tables MDX rendering fix works
• CI checks pass

🤖 Generated with Claude Code

[chaptersix]

Suggestion: rename -split to -subdir.

-split describes the transformation (splitting one file into many), but -subdir describes the output structure — "put this command's subcommands into a subdirectory." Someone reading -subdir cloud in a Makefile or CI workflow immediately understands what happens to the output, without needing to know the internal mechanics.

This also reads more naturally as the flag gets used alongside -output:

go run ./cmd/gen-docs \
-input internal/temporalcli/commands.yaml \
-input cliext/option-sets.yaml \
-input ../cloud-cli/temporalcloudcli/commands.yml \
-output dist/docs \
-subdir cloud

Also suggest updating the flag description from:

"Command name whose subcommands get separate files in a subdirectory (can be specified multiple times)"

to:

"Place subcommands of this command into a subdirectory instead of a single file (can be specified multiple times)"