build-generic: refactor and document job templates

Split the existing .workspace and .build jobs into the following:

- .workspace - handling git-repo and single-repository CI override
- .bitbake-workspace - handling bitbake customizations, including recipe CI
- .build-{linux,zephyr,freertos} - build a tailored set of recipes
- .build-recipe - build a single recipe
- .build-image - build a single recipe and collect the resulting image
- .build - (deprecated) - alias for .build-recipe

This should keep compatibility with downstream CI (meta-ohos and others)
but allow the docs repository to reuse the .workspace job entirely.

In addition, document all the jobs with reStructured Text, moving some
of the text from the implementation (yaml) files, into new documentation
chapters. Existing documentation is slightly re-structured to take the
new implementation details into account.

In last addition, the .workspace job is no longer strict about presence
of a directory called "sources/". This prevented re-using the job for
gitee builds.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>
14 jobs for !50 with feature/extra-targets in 52 minutes and 19 seconds (queued for 21 minutes and 57 seconds)
latest detached
Status Job ID Name Coverage
  Compliance
passed #15389
dco

00:00:14

 
  Build
passed #15402
build-docs

00:00:11

passed #15401
large-disk
freertos-armv5

00:02:01

passed #15392
large-disk
linux-qemu-arm

00:15:49

passed #15393
large-disk
linux-qemu-arm64

00:15:54

passed #15390
large-disk
linux-qemu-x86

00:15:41

passed #15391
large-disk
linux-qemu-x86_64

00:15:40

passed #15395
large-disk
linux-seco-imx8mm-c61

00:17:56

passed #15394
large-disk
linux-seco-intel-b68

00:17:28

passed #15396
large-disk
linux-stm32mp1-av96

00:36:07

passed #15400
large-disk
zephyr-96b-avenger

00:03:10

passed #15399
large-disk
zephyr-96b-nitrogen

00:03:26

passed #15398
large-disk
zephyr-qemu-cortex-m3

00:04:56

passed #15397
large-disk
zephyr-qemu-x86

00:04:41