doc: add documentation on porting from Buffer(arg) usage

[ChALkeR]

Refs: #19079

As previously discussed, it would be great if we could provide that instructions somewhere (docs, news, medium, whatever).

I made an initial draft in my repo here: Porting-Buffer.md, and @addaleax contributed significant parts of it.

What would be the next steps to discuss it and fix it so that it could be included somewhere?
I guess that needs to be done in time for 10.0 release.

Also: comments/patches to the doc at its current state are welcomed.

/cc @nodejs/tsc

[mcollina]

I think the best place is a medium blog post or a guide https://nodejs.org/en/docs/guides. Or both.

The deprecation of the Buffer constructor was documented here: https://github.com/nodejs/node/wiki/Breaking-changes-between-v5-and-v6.
The node docs also mark those methods are deprecated.
I went through that process for a code base and I think this was well documented.

[Trott]

/ping @nodejs/tsc

[Trott]

This is on the TSC agenda for this week, but it's not clear to me what the TSC needs to actually decide. Can we get a brief explanation of what is needed from the TSC on this? @ChALkeR

[Trott]

@nodejs/buffer