Website: Add blog post for 18.0.0

[raulcd]

Release blog post for 18.0.0.

Issues on the milestone are here: https://github.com/apache/arrow/milestone/64

[raulcd]

@jonkeane @paleolimbot @assignUser can you help with the R notes?

[jonkeane]

Suggested change

	## R notes
	For more on what’s in the 18.0.0 R package, see the [R changelog][4].
	## R notes
	* R functions that users write that use functions that Arrow supports in dataset
	queries now can be used in queries too. Previously, only functions that used
	arithmetic operators worked.
	For example, `time_hours <- function(mins) mins / 60` worked,
	but `time_hours_rounded <- function(mins) round(mins / 60)` did not;
	now both work. These are automatic translations rather than true user-defined
	functions (UDFs); for UDFs, see `register_scalar_function()`. [GH-41223](https://github.com/apache/arrow/issues/41223)
	* `mutate()` expressions can now include aggregations, such as `x - mean(x)`. [GH-41350](https://github.com/apache/arrow/issues/41350)
	* `summarize()` supports more complex expressions, and correctly handles cases
	where column names are reused in expressions. [GH-41223](https://github.com/apache/arrow/issues/41223)
	* The `na_matches` argument to the `dplyr::*_join()` functions is now supported.
	This argument controls whether `NA` values are considered equal when joining. [GH-41358](https://github.com/apache/arrow/issues/41358)
	* R metadata, stored in the Arrow schema to support round-tripping data between
	R and Arrow/Parquet, is now serialized and deserialized more strictly.
	This makes it safer to load data from files from unknown sources into R data.frames. [GH-41969](https://github.com/apache/arrow/issues/41969)
	* Turn on the S3 and ZSTD features by default for macOS. [GH-42210](https://github.com/apache/arrow/issues/42210)
	* Fix a bug in our implementation of `pull` on grouped datasets, it now
	returns the expected column. [GH-43172](https://github.com/apache/arrow/issues/43172)
	For full details of what’s in the 18.0.0 R package, see the [R changelog][4].

[amoeba]

@jonkeane these are the 17.0.0 changes though right?

Nic and I are working on a NEWS.md patch in apache/arrow#44496 and we can copy that in here in a bit.

[jorisvandenbossche]

I think just enumerating those issue links might not be that informative without more context (for someone reading this, you would have to click on all those links to have an idea what actually changed)

[raulcd]

I've updated some missing things on the blog post announcement. I'll merge the blog post to do the announcement. If someone wants to keep working on it (R notes for example) can be done on a follow up update.