doc: document the Alpha phase and v27 schedule

[RafaelGSS]

doc: document the Alpha phase and v27 schedule

Node.js 27 is the first release line to go through the Alpha phase
and to follow the new release schedule, with 30 months of Active LTS
and no Maintenance phase.

Refs: https://nodejs.org/en/blog/announcements/evolving-the-nodejs-release-schedule
Refs: https://github.com/nodejs/Release/issues/1187
Fixes: https://github.com/nodejs/Release/issues/1160

[MikeMcC399]

The table is missing an "Alpha Start" column and date for 27.x 2026-10-28.

As explained below, there are now 4 phases instead of three, so the table needs an additional column.
The column "Initial Release" could be renamed to "Current Start" to align with other column naming "x Start".

[aduh95]

Having a new table is probably the simplest

Suggested change

	| 27.x | **Alpha** | | 2027-04-22 | 2027-10-20 | - | 2030-04-30 |
	| Release | Status | Codename | Alpha Start | Current Start | LTS Start | End-of-life |
	| :--: | :---: | :---: | :---: | :---: | :---: | :---: |
	| 27.x | **Alpha** | | 2026-10-28 | 2027-04-22 | 2027-10-20 | 2030-04-30 |

[MikeMcC399]

Splitting the tables is a better suggestion.

[aduh95]

The two tables not being aligned is slightly dissatisfying, but looks like there's no easy way to fix that

[richardlau]

Should the new table go above the existing one?

[richardlau]

Should the new table go above the existing one?

I guess we're listing the releases in numerical order, so 22 is first. Maybe it should be the other way (i.e. the most recent release line first, which means reversing the existing table) but that is probably a separate discussion outside of this PR.

[MikeMcC399]

Adding sub-headings would make the table column width differences less disturbing.

I would support changing to a descending sort order for release lines (in a separate PR). That would make it like changelogs. More recent changes, being generally more relevant, come first.