docs: Update timeline for E15 alpha announcement (#30117)

* docs: Update timeline for E15 alpha announcement

* fix line break

Co-authored-by: Sofia Nguy <sofianguy@gmail.com>

docs/tutorial/electron-timelines.md

@@ -1,23 +1,27 @@
 # Electron Release Timelines
 
+Special notes:
+
 * The `-beta.1` and `stable` dates are our solid release dates.
 * We strive for weekly beta releases, however we often release more betas than scheduled.
 * All dates are our goals but there may be reasons for adjusting the stable deadline, such as security bugs.
 * Take a look at the [5.0.0 Timeline blog post](https://electronjs.org/blog/electron-5-0-timeline) for info about publicizing our release dates.
 * Since Electron 6.0, we've been targeting every other Chromium version and releasing our stable on the same day as Chrome stable. You can reference Chromium's release schedule [here](https://chromiumdash.appspot.com/schedule). See [Electron's new release cadence blog post](https://www.electronjs.org/blog/12-week-cadence) for more details on our release schedule.
+* Electron 15.0 only will include a special Alpha release. Starting in Electron 16.0, we will release on an 8-week cadence. See [Electron's new 8-week cadence blog post](https://www.electronjs.org/blog/8-week-cadence) for more details.
 
-| Version | -beta.1 | Stable | Chrome | Node |
-| ------- | ------- | ------ | ------ | ---- |
-| 2.0.0 | 2018-02-21 | 2018-05-01 | M61 | v8.9 |
-| 3.0.0 | 2018-06-21 | 2018-09-18 | M66 | v10.2 |
-| 4.0.0 | 2018-10-11 | 2018-12-20 | M69 | v10.11 |
-| 5.0.0 | 2019-01-22 | 2019-04-24 | M73 | v12.0 |
-| 6.0.0 | 2019-05-01 | 2019-07-30 | M76 | v12.4 |
-| 7.0.0 | 2019-08-01 | 2019-10-22 | M78 | v12.8 |
-| 8.0.0 | 2019-10-24 | 2020-02-04 | M80 | v12.13 |
-| 9.0.0 | 2020-02-06 | 2020-05-19 | M83 | v12.14 |
-| 10.0.0 | 2020-05-21 | 2020-08-25 | M85 | v12.16 |
-| 11.0.0 | 2020-08-27 | 2020-11-17 | M87 | v12.18 |
-| 12.0.0 | 2020-11-19 | 2021-03-02 | M89 | v14.16 |
-| 13.0.0 | 2021-03-04 | 2021-05-25 | M91 | v14.16 |
-| 14.0.0 | 2021-05-27 | 2021-08-31 | M93 | TBD |
+| Electron | Alpha | Beta | Stable | Chrome | Node |
+| ------- | ----- | ------- | ------ | ------ | ---- |
+| 2.0.0 | -- | 2018-Feb-21 | 2018-May-01 | M61 | v8.9 |
+| 3.0.0 | -- | 2018-Jun-21 | 2018-Sep-18 | M66 | v10.2 |
+| 4.0.0 | -- | 2018-Oct-11 | 2018-Dec-20 | M69 | v10.11 |
+| 5.0.0 | -- | 2019-Jan-22 | 2019-Apr-24 | M73 | v12.0 |
+| 6.0.0 | -- | 2019-May-01 | 2019-Jul-30 | M76 | v12.4 |
+| 7.0.0 | -- | 2019-Aug-01 | 2019-Oct-22 | M78 | v12.8 |
+| 8.0.0 | -- | 2019-Oct-24 | 2020-Feb-04 | M80 | v12.13 |
+| 9.0.0 | -- | 2020-Feb-06 | 2020-May-19 | M83 | v12.14 |
+| 10.0.0 | -- | 2020-May-21 | 2020-Aug-25 | M85 | v12.16 |
+| 11.0.0 | -- | 2020-Aug-27 | 2020-Nov-17 | M87 | v12.18 |
+| 12.0.0 | -- | 2020-Nov-19 | 2021-Mar-02 | M89 | v14.16 |
+| 13.0.0 | -- | 2021-Mar-04 | 2021-May-25 | M91 | v14.16 |
+| 14.0.0 | -- | 2021-May-27 | 2021-Aug-31 | M93 | TBD |
+| 15.0.0 | 2021-Jul-20 | 2021-Sep-01 | 2021-Sep-21 | M94 | TBD |