Renumber the README manual TOC after Extra themes was dropped (backport of #8089)

Backport of the TOC renumbering (PR #8089 by @samirpokharel, merged to quattro
as b86d4505) onto the v4-0-1 release branch.

Dropping manual/43-extra-themes.md shifted every chapter after it down by one,
on this branch as much as on quattro. The TOC rode along on the old numbers, so
its last ten links pointed at files that are not there: resolving every
manual/*.md link in this branch's README before the change found ten that do not
exist, from 43-extra-themes.md through 52-unattended-installs.md. After it, all
of them resolve.

Clean cherry-pick: the TOC now matches quattro's, so merging v4-0-1 into quattro
resolves without a conflict. test/shell passes: 189 files.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018DEMYa9UWtroz93DhMTtcV

README.md

@@ -61,19 +61,18 @@ its screenshots are also hosted.
 - [Prompt](manual/40-prompt.md)
 - [Branding](manual/41-branding.md)
 - [Common tweaks](manual/42-common-tweaks.md)
-- [Extra themes](manual/43-extra-themes.md)
-- [Making your own theme](manual/44-making-your-own-theme.md)
+- [Making your own theme](manual/43-making-your-own-theme.md)
 
 **The Rest**
 
-- [Mac support](manual/45-mac-support.md)
-- [Troubleshooting](manual/46-troubleshooting.md)
-- [FAQ](manual/47-faq.md)
-- [System snapshots](manual/48-system-snapshots.md)
-- [Security](manual/49-security.md)
-- [Omarchy on...](manual/50-omarchy-on.md)
-- [Dual Boot Install](manual/51-dual-boot-install.md)
-- [Unattended Installs](manual/52-unattended-installs.md)
+- [Mac support](manual/44-mac-support.md)
+- [Troubleshooting](manual/45-troubleshooting.md)
+- [FAQ](manual/46-faq.md)
+- [System snapshots](manual/47-system-snapshots.md)
+- [Security](manual/48-security.md)
+- [Omarchy on...](manual/49-omarchy-on.md)
+- [Dual Boot Install](manual/50-dual-boot-install.md)
+- [Unattended Installs](manual/51-unattended-installs.md)
 
 ## License