docs: link changelog entries to issues

[Markeli]

Adds issue links to the ## [Unreleased] CHANGELOG entries from #35 (HttpClient), #37 (RabbitMQ.Client 7) and #38 (RabbitMQ request processing options):

• Mailgun, UnisenderGo, Iqsms, Smsc → Email/SMS HTTP providers: default timeout, no retries of sends, Smsc error mapping and logging #82, Mailgun: make the sender usable and map HTTP errors correctly #83, Curiosus.EMail.Mailgun: sender can't be created, MailgunEmailOptions.AssertValid throws NotImplementedException #12, Iqsms: move to the JSON gateway and take credentials out of the URL #84
• Tools.Web → Tools.Web: finish ReCaptcha client, small fixes in SiteMap and PreviewDetector #67
• RabbitMQ → RabbitMQ: slim and extend the Curiosus.RabbitMQ RPC client (shared connection, reply modes, ConnectionUri) #75, RabbitMQ: learning sandbox for RPC reply modes in samples/RabbitMQ.Sandbox #76
• RequestProcessing.RabbitMQ → RequestProcessing.RabbitMQ: engine adapter on the Curiosus.RabbitMQ consumer host, with correct acks and RPC responder mode #77, RabbitMQ request processing sample fails at startup: missing ClientName #18

Wording is unchanged; no release.

🤖 Generated with Claude Code

[github-actions]

Release check

Package	Version	On merge
Curiosus.Tools.Web	2.0.0	📝 changelog entry, released later
Curiosus.SMS.Smsc	2.0.0	📝 changelog entry, released later
Curiosus.SMS.Iqsms	2.0.0	📝 changelog entry, released later
Curiosus.RabbitMQ	2.0.0	📝 changelog entry, released later
Curiosus.RequestProcessing.RabbitMQ	2.0.0	📝 changelog entry, released later
Curiosus.EMail.UnisenderGo	2.0.0	📝 changelog entry, released later
Curiosus.EMail.Mailgun	2.0.0	📝 changelog entry, released later

_{For 475e9b0.}

[Markeli]

Moved into the release PR #99 (commit 95adcc4), so the links land with the 3.0.0/2.1.0 sections.