FINERACT-2336: improve asciidoc post-1.12.1 release

[meonkeys]

Description

See FINERACT-2336 and detailed change descriptions in the commit log message.

Please have a look at the rendered HTML and PDF outputs, and compare them with https://fineract.apache.org/docs/current/.

Please pay special attention to changes in "Step 13: Close Release Branch"--there's an open discussion about how to do this properly. The steps work as indicated in this patch, but it eschews a flat commit trail in favor of simplicity while closing a release branch.

Checklist

Please make sure these boxes are checked before submitting your pull request - thanks!

• Write the commit message as per https://github.com/apache/fineract/#pull-requests
• Acknowledge that we will not review PRs that are not passing the build ("green") - it is your responsibility to get a proposed PR to pass the build, not primarily the project's maintainers.
• Create/update unit or integration tests for verifying the changes made.
• Follow coding conventions at https://cwiki.apache.org/confluence/display/FINERACT/Coding+Conventions.
• Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes
• Submission is not a "code dump". (Large changes can be made "in repository" via a branch. Ask on the developer mailing list for guidance, if required.)

[meonkeys]

Huh, that was an odd test failure:

  org.apache.fineract.integrationtests.datatable.DatatableIntegrationTest
  
    Test validateCreateReadDeleteDatatableWithCaseSensitive() PASSED
    Test validateInsertNullValues() PASSED
    Test validateCreateAndEditDatatable() PASSED
    Test validateCreateReadDeleteDatatable() PASSED
    Test validateReadDatatableMultirow() FAILED
  
    org.opentest4j.AssertionFailedError: expected: <1582-10-13 11:16:23> but was: <1582-10-23 11:16:23>
        at app//org.apache.fineract.integrationtests.datatable.DatatableIntegrationTest.validateReadDatatableMultirow(DatatableIntegrationTest.java:810)

I'll see if I can repro that locally.

I'm also going to close this and create a new PR merging from my fork instead of from a branch in this repo (to prevent duplicate jobs in github actions CI).