Build the docs if there are changes in docs and don't run other tasks if the changes are only in docs

Hi,

There is an old work item about building the docs if there are changes
in the docs, otherwise don't build the docs. I wanted to make an
addition to that idea; if the changes are only in the docs, don't run
all tasks except building the docs task; this could help to save more
CI times. I attached two patches.

I assumed that the docs related changes are limited with the changes
in the docs folder but I am not really sure about that.

v1-0001-Only-built-the-docs-if-there-are-changes-are-in-t.patch:
This patch creates another task named 'Building the Docs' and moves
building the doc script from 'CompilerWarnings' task to this task.
This building the docs task only runs if there are changes in the docs
(under the doc/**) or in the CI files ('.cirrus.yml',
'.cirrus.tasks.yml') and if a specific OS is not requested.

v1-0002-Just-run-the-Build-the-Docs-task-if-the-changes-a.patch:
This patch adds that: if the changes are *only* in the docs (under the
doc/**), *only* run building the docs task.

As a summary:
1- If the changes are not in the docs: Don't run build the docs task.
2- If the changes are in the docs or in the CI files : Run build the docs task.
3- If the changes are only in the docs: Only run build the docs task.
4- If 'ci-os-only:' set (There could be changes in the docs): Don't
run build the docs task.

Any kind of feedback would be appreciated.

Regards,
Nazir Bilal Yavuz
Microsoft