Refresh the AI features settings page (content, role, and filename) to match the current product

## Summary

The page [`explore-analyze/ai-features/manage-access-to-ai-assistant.md`](https://github.com/elastic/docs-content/blob/main/explore-analyze/ai-features/manage-access-to-ai-assistant.md) has drifted from what it actually documents. It started life as an "AI Assistant access" page, but it has grown into the general **AI features settings** page: it now covers **GenAI Settings**, **Feature Settings**, model selection, and (via #7495) token usage tracking. The filename, URL slug, framing, and some prose no longer reflect that role.

This issue tracks a focused refresh so the page's content, structure, and filename match the current state and use of these settings in the product.

## Why

- The H1 is already **"Manage access to AI features"**, but the filename/slug is still `manage-access-to-ai-assistant` — a mismatch that's confusing to readers and authors. "Manage access" also undersells the page, which is really about *configuring* AI features, not just controlling access.
- The page accumulated content across several releases without a holistic pass, so its structure, terminology, and voice are inconsistent in places.

## Proposed work

### 1. Filename / slug + framing
- [ ] Rename to a clearer slug, for example `configure-ai-features.md` or `ai-features-settings.md` (decide on the preferred term).
- [ ] Add a redirect from the old path (see the docs-redirects skill / `redirects.yml`).
- [ ] Update all inbound links across `docs-content` (grep for `manage-access-to-ai-assistant`) and check cross-repo `://` links.
- [ ] Revisit the H1 and `navigation_title` so they match the page's actual scope.

### 2. Structure / narrative
- [ ] Reorganize the page so the narrative works for its current role — settings grouped logically (for example GenAI Settings vs. Feature Settings vs. token usage tracking), with an opening that frames the page as *configuring AI features* rather than *managing access*.
- [ ] If the page is trying to be several things at once, consider whether some content should move to (or be split into) more appropriately scoped pages, with cross-references.

### 3. Content accuracy / role
- [ ] Confirm each setting (GenAI Settings, Feature Settings, and their sub-settings) still matches the current UI labels and behavior at HEAD, across the versions the page serves (`stack: ga 9.2+`, `serverless: ga`).
- [ ] Confirm the page's content type still fits its broadened role (settings/reference vs. the older "manage access" how-to framing).

### 4. Style cleanup (from a Vale + manual review on 2026-07-30)

These are pre-existing issues, deliberately left out of #7495 to keep that PR scoped:

- **Word choice:** `choose` → `select` (the AI Assistant visibility setting, the "choosing one" cross-reference, and "Choose a default model"); "main menu" → "navigation menu" (Feature Settings intro).
- **Tense/voice:** "AI-powered features **will default** to…" → present tense ("default to…"); avoid "should power it" → "powers it".
- **Toggle wording:** prefer "turn on/off" over enable/disable for settings and toggled features ("Enable it to prevent…", "Default setting: disabled", "features are enabled").
- **Consistency:** align the two app-navigation lead-ins ("by using the navigation menu or the global search field" vs. "in the main menu or use the global search field"); align the two variants of the AI Agent/Assistant cross-reference lead-in.
- **Capitalization:** "Space's" is capitalized in the AI feature visibility bullets but lowercase elsewhere — use lowercase "space" unless matching a UI label.
- **Formatting:** the GenAI Settings (9.2–9.3) image block is missing the `:screenshot:` attribute the other three image blocks use; add it or confirm the omission is intentional. Remove trailing whitespace after the privilege link in Requirements.

## Context

Split out of #7495 (token usage tracking documentation) to keep that PR scoped to the new content. Do this as its own PR.

Start with explore-analyze/ai-features/manage-access-to-ai-assistant.md and review the docs-redirects skill, redirects.yml, and inbound-link search for the old slug. Compare the page with the current UI labels and behavior for stack ga 9.2+ and serverless ga. Done means the renamed, restructured page, redirect, links, terminology, and formatting are consistent and scoped in a separate PR.

Written by the indexing model from the issue text.