Out of date examples in docs that call render() with a Context instance instead of regular dict

[Alasdair Nicol]

Following #30906, I found a couple more examples in the docs that are calling the template's render method with a Context instance when they should use a regular dict.

• ​docs/ref/contrib/sites.txt - I will open a pull request for this
• ​howto/custom-template-tags.txt - I am not sure how to rewrite this example.

I found these by searching with the following commands:

cd docs
git grep 'render.*Context'
git grep 'from django.*import.*Context'

Those grep commands also return results from ref/templates/api.txt, but they are ok because they use t = Template() instead of loader.get_template().