Adds BUILD_DOCS flag as opt-in flag.

[francocipollone]

Related to https://github.com/ToyotaResearchInstitute/maliput_infrastructure/issues/244 Related to https://github.com/ToyotaResearchInstitute/maliput_documentation/issues/81

[francocipollone]

@agalbachicar 1 - Once this PR is merged I will proceed to extend this changes to the rest of the repositories 2 - I haven't added a workflow that build the docs and verifies that no warnings are generated. I am thinking what to be the best way to do that.

[francocipollone]

I've pushed a commit that modifies the build.yaml workflow so:

• When creating the PR the workflow will be run passing the BUILD_DOCS=Off so, the documentation won't be built.
• In case build are wanted to be built (and test), it should be manually executed via workflow_dispatch event. When doing this the value of the BUILD_DOCS flag can be selected to be "On" or "Off" See next image:

It is true that documentation errors won't be automatically analyzed however this way CI time isn't duplicated .

@agalbachicar wdyt?