DOCS-1827 - Consolidate Query Assist into Search Autocomplete

[kimsauce]

Purpose of this pull request

Retires the standalone Query Assist doc, which was never a UI-labeled feature, and merges its content into the existing Search Autocomplete page. Adds cross-link notes so Search Autocomplete, AI Parse Assist, and Mobot's Log Analysis Agent (three easily-confused AI-assisted search capabilities) are clearly distinguished from one another. Fixes two dead release-note links that pointed at the retired page, adds a redirect for the old URL, and removes orphaned/stale screenshots. Also includes an unrelated small fix: shrinks the account-preferences icon and moves it inline with its instruction text in two other docs.

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

https://sumologic.atlassian.net/browse/DOCS-1827

[JV0812]

Heads-up: cross-PR conflict with #7058

While reviewing #7058 ("DOCS-1798 - Cross-link Mobot on high-traffic search docs"), I noticed it modifies docs/search/query-assist.md (uncommenting a Mobot callout there), while this PR deletes that same file entirely as part of consolidating it into search-autocomplete.md.

Whichever of these two PRs merges second should account for the other:

• If this PR (DOCS-1827 - Consolidate Query Assist into Search Autocomplete #7061) merges first, DOCS-1798 - Cross-link Mobot on high-traffic search docs #7058's edit to query-assist.md becomes moot (file gone).
• If DOCS-1798 - Cross-link Mobot on high-traffic search docs #7058 merges first, this PR needs to make sure the new Mobot callout isn't silently dropped when query-assist.md's content is folded into search-autocomplete.md.

Just flagging for merge sequencing, not a blocker on either PR's own content.