doc: Add doc/develop/release_cycle.rst

Migrate the RelaseCycle wiki page to Sphinx.  In terms of visible
changes, we stop having a dynamic countdown to when the release is.  And
we drop the year-based statistics, that were not being kept up to date.
For the moment, we only link to statistics for v2022.07 but will add
back the historical data in a subsequent patch.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>

doc/develop/index.rst

@@ -12,6 +12,7 @@ General
    codingstyle
    designprinciples
    process
+   release_cycle
 
 Implementation
 --------------

doc/develop/release_cycle.rst

@@ -0,0 +1,94 @@
+Release Cycle
+=============
+
+The U-Boot projects attempts to maintain a fixed, predictable Release
+Cycle as follows:
+
+* We will have U-Boot releases at a fixed release interval of (approximately)
+  every 3 months.
+
+* Under normal conditions the release date will be the first Monday of the month.
+
+* Immediately following each release, there will be a "merge window" of
+  normally 21 days, i. e. if the release was on a Monday, then the merge window
+  will close on the Monday in the 3rd week after the release.
+
+  While this merge window is open, new features can be added to the U-Boot
+  source tree. This is further expanded on in the :doc:`process`.
+
+* After the merge window closes, no new features may be added to allow for a
+  release candidate phase which is intended to fix bugs and regressions.
+
+*Note:* While we try to adhere to the release schedule, we will
+not hesitate and take the liberty to delay a release if there are
+good reasons, for example if there are known bugs or other technical
+reasons.  The code will be released when it is considered ready
+without caring too much about the announced deadline.
+
+*Note 2:* Even though we follow Linux ways in many respects, there are
+differences in the actual procedures, which are documented in the
+:doc:`process`.
+
+Version Numbers
+---------------
+
+Starting with the release in October 2008, the names of the releases were
+changed from numerical release numbers without deeper meaning into a time stamp
+based numbering.  Regular releases are now identified by names consisting of
+the calendar year and month of the release date.  Additional fields are
+frequently used to denote the release candidates.  They are also used on rare
+occasions to note a bug fix release on top of the previous stable release.
+
+Examples::
+
+  U-Boot v2009.11	- Release November 2009
+  U-Boot v2009.11.1	- Bug fix release 1 on top of the November 2009 release
+  U-Boot v2010.09-rc1   - Release candidate 1 for September 2010 release
+
+Current Status
+--------------
+
+* U-Boot v2022.07 was released on Mon 11 July 2022.
+
+* The Merge Window for the next release (v2022.10) is **open** until -rc1
+  release on Mon 25 July 2022.
+
+* Release "v2022.10" is scheduled for 03 Oct 2022.
+
+Future Releases
+---------------
+
+.. The following commented out dates are for when release candidates are
+   planned to be tagged.
+
+.. For the next scheduled release, release candidates were made on::
+
+.. * U-Boot v2022.10-rc1 was released on Mon 25 July 2022.
+
+.. * U-Boot v2022.10-rc2 was released on Mon 08 August 2022.
+
+.. * U-Boot v2022.10-rc3 was released on Mon 22 August 2022.
+
+.. * U-Boot v2022.10-rc4 was released on Mon 05 September 2022.
+
+.. * U-Boot v2022.10-rc5 was released on Mon 19 September 2022.
+
+Please note that the following dates are planned only and may be deviated from
+as needed.
+
+* "v2022.10": end of MW = Mon, Jul 25, 2022; release = Mon, Oct 03, 2022
+
+* "v2023.01": end of MW = Mon, Oct 24, 2022; release = Mon, Jan 09, 2023
+
+* "v2023.04": end of MW = Mon, Jan 30, 2022; release = Mon, Apr 03, 2023
+
+* "v2023.07": end of MW = Mon, Apr 24, 2023; release = Mon, Jul 03, 2023 
+
+Previous Releases
+-----------------
+
+Note: these statistics are generated by our fork of `gitdm
+<https://source.denx.de/u-boot/gitdm>`_, which was originally created by
+Jonathan Corbet.
+
+* :doc:`statistics/u-boot-stats-v2022.07` which was released on 11 July 2022.