Docs: Google Sheets quarterly update (FY27 Q3)

[lwandz13]

Quarterly documentation update (FY27 Q3) for the Google Sheets data source. This is an accuracy-focused refresh: all content was reconciled against the plugin's Go backend and TypeScript frontend, review dates were updated, one new behavior (from plugin 2.6.0) was documented, and prose was brought in line with the Grafana Writer's Toolkit / Vale style. All seven pages now pass Vale with zero errors.

_index.md:
Updated review date, converted en-dash list separators to colons in "Get started", present-tense fix in the quick start, converted smart quotes to straight quotes

configure.md:
Updated review date, corrected the Drive API scope from drive.metadata.readonly to drive.readonly to match the scope the plugin actually requests, present-tense fixes in the default spreadsheet ID and JWT sections, code-formatted camelCase

query-editor.md:
Updated review date, added a "Formula errors in numeric columns" note to "How data is interpreted" (cells with #DIV/0!, #N/A, #REF! now yield a null value), present-tense fix, converted smart quotes to straight quotes

template-variables.md:
Updated review date, present-tense fixes, converted en-dash separators to colons, reworded to avoid the Grafana's possessive

annotations.md:
Updated review date, converted en-dash separators to colons, datetime to date-time

alerting.md:
Updated review date, present-tense fix, converted en-dash separators to colons in use cases and next steps

troubleshooting.md:
Updated review date, added a "Null or empty values for cells with formula errors" entry documenting the 2.6.0 behavior change (formula errors in numeric columns no longer fail the whole query), converted smart quotes to straight quotes

Technical accuracy verified against:

• src/plugin.json (supported features, grafanaDependency >=11.6.0-0, plugin ID)
• pkg/googlesheets/googleclient.go (auth flow, requested OAuth scopes, TestClient behavior, error messages)
• pkg/googlesheets/googlesheets.go (numeric-cell null handling from PR Fix panic in numberConverter on nil EffectiveValue #641 / 2.6.0)
• pkg/googlesheets/datasource.go (CheckHealth messages)
• pkg/models/settings.go, pkg/models/query.go (provisioning jsonData keys, query fields)
• src/components/ConfigEditor.tsx, QueryEditor.tsx, VariableQueryEditor.tsx (auth types, cache options, query and variable fields)