docs: correct the community sharing, voice mode prompt and prompt template entries in the env reference

[silentoplayz]

Summary

Corrects eight entries in the environment variable reference against the current code:

• ENABLE_COMMUNITY_SHARING: drops the "Evaluation Feedbacks" bullet. Feedbacks.svelte still defines a share handler, but nothing calls it, so there is no "Share to Open WebUI Community" button under Evaluations for this setting to show or hide.
• ENABLE_VOICE_MODE_PROMPT: the admin setting is now labeled Voice Mode Prompt, not "Voice Mode Custom Prompt".
• VOICE_MODE_PROMPT_TEMPLATE: the default is an empty string, which falls back to the built-in voice prompt. DEFAULT_VOICE_MODE_PROMPT_TEMPLATE is a constant in config.py, not an environment variable. The wording matches the existing TITLE_GENERATION_PROMPT_TEMPLATE entry.
• FOLLOW_UP_GENERATION_PROMPT_TEMPLATE, TAGS_GENERATION_PROMPT_TEMPLATE, QUERY_GENERATION_PROMPT_TEMPLATE and TOOLS_FUNCTION_CALLING_PROMPT_TEMPLATE: same correction. Each defaults to an empty string and falls back to the built-in template shown below it, and none of the DEFAULT_... names is an environment variable.
• RAG_TEMPLATE: its default really is the built-in template, so only the "environment variable" wording changes.

Related issue or discussion

None.

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers

Checked against dev at 015dbc8: shareHandler in src/lib/components/admin/Evaluations/Feedbacks.svelte has no caller, the settings.admin.interface.voiceModePrompt.label translation reads "Voice Mode Prompt", and config.py reads VOICE_MODE_PROMPT_TEMPLATE, FOLLOW_UP_GENERATION_PROMPT_TEMPLATE, TAGS_GENERATION_PROMPT_TEMPLATE, QUERY_GENERATION_PROMPT_TEMPLATE and TOOLS_FUNCTION_CALLING_PROMPT_TEMPLATE with a default of '' and RAG_TEMPLATE with DEFAULT_RAG_TEMPLATE. The fallback to the built-in template on an empty value is in routers/tasks.py (follow-up, tags, query) and utils/middleware.py (tools).