docs: Update the announcement email template to ensure that disclaimers have been added

[Xuanwo]

As mentioned in the discussion at https://lists.apache.org/thread/0d8dxxbhb28m7w7kqxs1d2n0hsdybocg, Sebb reminded us to include disclaimers in our announcement.

We should add the following content:

---
Apache OpenDAL (incubating) is an effort undergoing incubation at the Apache
Software Foundation (ASF), sponsored by the Apache Incubator PMC.

Incubation is required of all newly accepted projects until a further review
indicates that the infrastructure, communications, and decision making process
have stabilized in a manner consistent with other successful ASF projects.

While incubation status is not necessarily a reflection of the completeness
or stability of the code, it does indicate that the project has yet to be
fully endorsed by the ASF.

[Venderbad]

pls assign to me

[dqhl76]

Have fun.