Explicitly mention setting defaults is for insiders only (meta plugin)

As the meta plugin is reserved for insiders only, this section could also mention that so people don't run into the below error.

```
$ mkdocs serve
ERROR   -  Config value 'plugins': The "meta" plugin is not installed

Aborted with 1 configuration errors!
```
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Santhosh Kumar Srinivasan 2024-01-06 17:08:24 +05:30 committed by GitHub
parent 157c6dbd32
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@ -535,6 +535,11 @@ This will disable automatic reading time computation.
#### Setting defaults
<!-- md:sponsors -->
<!-- md:version insiders-4.21.0 -->
<!-- md:plugin [meta] built-in -->
<!-- md:flag experimental -->
If you have a lot of posts, it might feel redundant to define all of the above
for each post. Luckily, the [built-in meta plugin] allows to set default front
matter properties per folder. You can group your posts by categories, or