Publish roll-up announcement for the Cosmos DB Functions quickstart trio (#877, #878, #879)

## Goal

Once #877 (JavaScript), #878 (Java), and #879 (PowerShell) merge, publish a single announcement covering the new Cosmos DB Functions quickstarts together, instead of three separate template-add notices.

## Why this matters

The three PRs cover one cohesive customer story: a Functions plus Cosmos DB language matrix authored by @paulyuk. A consolidated announcement gives developers a clearer picture of what shipped and why it matters, keeps the gallery-update feed cleaner, and sets a reusable pattern for similar releases ahead (the Service Bus matrix in #832 through #836, the Timer trigger set in #844, and the Flex plus Files mount in #847).

## Scope

- [ ] Confirm #877, #878, and #879 have merged
- [ ] Draft a short announcement and pick the right surface (blog post, social, or release note) that:
- Names the three new quickstarts and their target audience
- Links each template's source repo
- Frames the language matrix as a single release
- Credits @paulyuk
- [ ] Coordinate publishing channel and timing with the Functions DevRel team

## Out of scope

- Per-template announcements
- Other Functions PRs still open (#832, #844, #847), which deserve their own roll-ups once they merge

## Context

Committed to in PR comments on #877, #878, and #879.

Start by confirming that #877, #878, and #879 have merged. Then review the three quickstarts and the committed PR comments, draft one announcement covering their shared Functions and Cosmos DB language matrix, and coordinate the channel and timing with the Functions DevRel team. Done means the consolidated announcement is published with the three quickstarts, source links, target audience, and @paulyuk credited.

Written by the indexing model from the issue text.