[wwwdocs] Add Ada's changelog entry

[Fernando Oleo Blanco]

Hi,

this is my first patch to GCC, if there is anything off, please, say
so. I have used the default HTML formatting that comes with Emacs. I
have created the patch using the `git format-patch` utility.

One thing that may not be allowed (I am not aware of any rule against
it but still) is the amount of nesting in the lists. The section "Ada
2022 extensions" has list that nest up to "level 3". I am pointing this
out, since no other section has 3 levels of nesting, all have at most 2.

I have also added a formatted code example. I do not know if this is
allowed. Trying to explain Pattern Matching without a minimal example
is not something that I am capable of.

The patch contains verbatim expressions from
https://blog.adacore.com/going-beyond-ada-2022 but Arnaud has provided
explicit permission to do so; he is also a GCC contributor.

I have not signed any legal requirements to contribute to GCC, however,
this is just a documentation patch. I am not providing any content
that is new or improving a part of GCC, so this should not be necessary.

Regards,
--
Fernando Oleo Blanco
https://irvise.xyz
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