chore(www): stop tracking auto-generated rss.xml (#45745)

## Summary

`apps/www/public/rss.xml` is committed to the repo AND rewritten on
every `pnpm dev`, `pnpm build`, and `pnpm typecheck` run via
`content:build` -> `apps/www/scripts/generateStaticContent.mjs`. Anyone
working in `apps/www` (or running `pnpm typecheck` from the monorepo
root) sees a phantom `M apps/www/public/rss.xml` in `git status` every
session. Easy to accidentally commit, otherwise has to be repeatedly
stashed.

The sibling `changelog-rss.xml` (written by the same script) is already
gitignored. This aligns the main blog rss with the same treatment.

## Changes

- Add `/public/rss.xml` to `apps/www/.gitignore` (alongside the existing
changelog-rss entries; renamed the section header from "Changelog
generated feeds" to "Generated feeds" to reflect what it now covers).
- `git rm --cached apps/www/public/rss.xml` so git stops tracking the
file. The on-disk copy is preserved for local dev.

## Testing

No behavior change — the file is still generated by `content:build` and
served from `public/` at build time.

- [x] `git check-ignore -v apps/www/public/rss.xml` resolves to
`apps/www/.gitignore:36`
- [x] `apps/www/public/rss.xml` still on disk after `git rm --cached`
- [ ] On a fresh clone + `pnpm install + pnpm --filter www build`, the
file regenerates and is served at `/rss.xml` on the resulting build
(Vercel preview will confirm via deploy).

## Linear

- fixes GROWTH-836

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Updated repository configuration to properly manage auto-generated
files.

[![Review Change
Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/supabase/supabase/pull/45745)

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/www/.gitignore

@@ -32,7 +32,8 @@ public/sitemap_www.xml
 # Generated .md content bundle (built by scripts/generateMdContent.mjs)
 app/api-v2/md/content.generated.ts
 
-# Changelog generated feeds and markdown (built by content:build)
+# Generated feeds and markdown (built by content:build)
+/public/rss.xml
 /public/changelog-rss.xml
 /public/changelog-rss
 /public/changelog.md