Add changelog entry for Markdown features in Ddoc

[dgileadi]

Note that this references the changed documentation in this dlang.org PR.

Also I hope this is the way to make a changelog entry—please let me know if I got it wrong.

[dlang-bot]

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Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#9645"

[PetarKirov]

We're not yet building the pages on dlang.org with -preview=markdown.

@wilzbach, what is necessary to change this?

[wilzbach]

Enabling it for dlang.org.

[wilzbach]

Hmm, we typically don't use dots in the title (this is the menu item and will be used in the generated changelog TOC).