Add sendai03

[thatblue]

仙台Ruby会議03用の転送設定の追加をお願いいたします。

リダイレクト先: https://sendairb.github.io/sendairk03/
Rubyの会 issue: ruby-no-kai/official#566

[sentry]

Bug: The location /sendai03 proxies to the wrong upstream path. The content is at /sendairk03, not /sendai03, which will cause 404 errors for users.
_{Severity: HIGH}

Suggested Fix

To fix the 404 error, change the location directive from location /sendai03 to location /sendairk03. This aligns the request path with the actual content path on the GitHub Pages server, as indicated by the proxy_redirect line.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: config/nginx.conf.erb#L460-L465

Potential issue: The nginx configuration for `sendai03` has a path mismatch. The
`location /sendai03` block, combined with `proxy_pass https://sendairb.github.io;`, will
forward requests to `https://sendairb.github.io/sendai03`. However, the `proxy_redirect`
directive in the same block indicates the correct upstream path is
`https://sendairb.github.io/sendairk03`. This inconsistency means any user navigating to
`regional.rubykaigi.org/sendai03` will receive a 404 error because the requested path
does not exist on the GitHub Pages server. This pattern is inconsistent with other
working configurations in the file.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[sorah]

あー、rewriteしないと確かに404になるかも? @thatblue