ICU-21349 Import 'Updating Measure Unit' sites page, improve formatting.

Sites page was last updated on: 21 Oct 2020.

See #1480

docs/processes/release/tasks/updating-measure-unit.md

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+# Updating Measure Unit with new CLDR data
+
+This document explains how to update the C and JAVA version of the MeasureUnit
+class with new CLDR data.
+
+Code is generated by running MeasureUnitTest.java unit tests, which writes
+generated code to System.out. Two ways to access this:
+
+1. Within **eclipse**:
+   - Open MeasureUnitTest.java, run it by clicking on the green play button on
+     menu bar.
+   - Copy the generated code from the eclipse console to the clipboard.
+
+2. With **ant**:
+   - Run: `ant checkTest
+     -Dtestclass='com.ibm.icu.dev.test.format.MeasureUnitTest'`
+   - Open the checkTest output: `out/junit-results/checkTest/html/index.html`
+   - Navigate to "System.out" at the bottom of the MeasureUnitTest page to find
+     the generated code, and copy to the clipboard.
+
+After syncing CLDR data with ICU do the following. This documentation assumes
+that you are updating the MeasureUnit clases for ICU 68.
+
+* Check out
+  $GIT_ROOT/icu4j/main/tests/core/src/com/ibm/icu/dev/test/format/MeasureUnitTest.java
+* Open MeasureUnitTest.java.
+* Find the `testZZZ` test, its code should all be commented out. This test will
+  execute last and will run the desired code.
+
+    Make sure DRAFT_VERSIONS at top of MeasureUnitTest.java is set correctly.
+    These are the ICU versions that have draft methods.
+
+## Update MeasureUnit.java
+
+* Change `testZZZ` to run `generateConstants(“68”); // ICU 68.`
+* Run MeasureUnitTest.java, copy the generated code (see instructions above).
+* Open MeasureUnit.java:
+  $GIT_ROOT/icu4j/main/classes/core/src/com/ibm/icu/util/MeasureUnit.java
+* Look for line containing:
+
+  `// Start generated MeasureUnit constants`
+* Look for line containing:
+
+  `// End generated MeasureUnit constants`
+* Replace all the generated code in between with the contents of the clipboard
+* Run the MeasureUnitTest.java to ensure that the new code is backward
+  compatible. These compatibility tests are called something like
+  `TestCompatible65`, which tests backward compatibility with ICU 65.
+* Create a compatibility test for ICU 68. Change `testZZZ` to run
+  `generateBackwardCompatibilityTest(“68”)`
+* Run tests.
+* Copy generated test (see instructions above) into MeasureUnitTest.java
+* Run tests again to ensure that new code is backward compatible with itself
+
+## Update ICU4C
+
+* checkout ICU4C
+
+### Update measunit.h
+
+* Change testZZZ to run `generateCXXHConstants(“68”); // ICU 68`.
+* Run MeasureUnitTest.java, copy the generated code (see instructions above).
+* Open $GIT_ROOT/icu4c/source/i18n/unicode/measunit.h. Look for line containing:
+
+  `// Start generated createXXX methods`
+* Look for line:
+
+  `// End generated createXXX methods`
+* Replace all the generated code in between with the contents of the clipboard
+
+### Update measunit.cpp
+
+* Change testZZZ to run generateCXXConstants();
+* Run MeasureUnitTest.java, copy the generated code (see instructions above).
+* Open $GIT_ROOT/icu4c/source/i18n/measunit.cpp. Look for line containing:
+
+  `// Start generated code for measunit.cpp`
+* Look for lines
+
+  `// End generated code for measunit.cpp`
+* Replace all the generated code in between with the contents of the clipboard
+
+\# Update measfmt.cpp -- Update the value for `MEAS_UNIT_COUNT` - no longer
+necessary
+
+### Run C++ tests
+
+* Run `./intltest format/MeasureFormatTest` from `test/intltest` to ensure new
+  code is backward compatible.
+* Create a compatibility test for ICU 68. Change `testZZZ` in eclipse to run
+  `generateCXXBackwardCompatibilityTest(“68”)`
+* Run tests.
+* Copy generated test (see instructions above) into
+  $GIT_ROOT/icu4c/source/test/intltest/measfmttest.cpp. Make other necessary
+  changes to make test compile. You can find these changes by searching for
+  `TestCompatible65()`
+* Run tests again to ensure that new code is backward compatible with itself
+
+## Finishing changes
+
+These last changes are necessary to permanently record the ICU version number of
+any new measure units. Without these changes any new functions for this release
+will be considered new for the next release too.
+
+* Change `testZZZ` to run `updateJAVAVersions(“68”);`
+* Run MeasureUnitTest.java, copy the generated code (see instructions above).
+* Append the clipboard contents to the values of the JAVA_VERSIONS variable
+  near the top of MeasureUnitTest.java.
+
+  **Important:** what you are copying are just the new functions for the current
+  ICU version, in this case 68. Therefore append, do not replace.