feat: support DKIM (#129)

vitepress-docs/docs/zh/guide/dkim.md

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+# 配置 DKIM
+
+参考: [Adding-a-DKIM-Signature](https://support.mailchannels.com/hc/en-us/articles/7122849237389-Adding-a-DKIM-Signature)
+
+Creating a DKIM private and public key:
+Private key as PEM file and base64 encoded txt file:
+
+```bash
+openssl genrsa 2048 | tee priv_key.pem | openssl rsa -outform der | openssl base64 -A > priv_key.txt
+```
+
+Public key as DNS record:
+
+```bash
+echo -n "v=DKIM1;p=" > pub_key_record.txt && \
+openssl rsa -in priv_key.pem -pubout -outform der | openssl base64 -A >> pub_key_record.txt
+```
+
+在 `Cloudflare` 的 `DNS` 记录中添加 `TXT` 记录
+
+例如:
+
+- `_dmarc`: `v=DMARC1; p=none; adkim=r; aspf=r;`
+- `mailchannels._domainkey`: `v=DKIM1; p=<content of the file pub_key_record.txt>`
+
+那我在 `wrangler.toml` 中的配置应该是这样的:
+
+```toml
+DKIM_SELECTOR = "mailchannels"
+DKIM_PRIVATE_KEY = "<priv_key.txt 的内容>"
+```