最近2周，文档服务器访问不稳定：偶尔可以访问，大多数时候不能访问。
以下是我从阿里云服务器上curl的信息：
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# curl https://docs.espressif.com
curl: (60) SSL certificate problem: certificate has expired
More details here: http://curl.haxx.se/docs/sslcerts.html
curl performs SSL certificate verification by default, using a "bundle"
of Certificate Authority (CA) public keys (CA certs). If the default
bundle file isn't adequate, you can specify an alternate file
using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
the bundle, the certificate verification probably failed due to a
problem with the certificate (it might be expired, or the name might
not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
the -k (or --insecure) option.
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头疼。