doc/getting-started: point out where to run make

[maribu]

Contribution description

This improves documentation at two locations:

1. The getting started section previously never stated that make is supposed to be run in an applications folder
2. The help message when make in the root directory got a tl'dr section at the end

Testing procedure

1. The CI will add a link to the doc
2. Run make in the root directory of the repo. The output should be:

make
Welcome to RIOT - The friendly OS for IoT!

You executed 'make' from the base directory.
Usually, you should run 'make' in your application's directory instead.

Please see our Quick Start Guide at:
    https://doc.riot-os.org/getting-started.html
You can ask questions or discuss with other users on our forum:
    https://forum.riot-os.org

Available targets for the RIOT base directory include:
 generate-{board,driver,example,module,pkg,test}
 info-{applications,boards,emulated-boards} info-applications-supported-boards
 print-versions
 clean distclean pkg-clean
 doc doc-{man,latex}

==> tl;dr Try running:
    cd examples/default
    make BOARD=<INSERT_BOARD_NAME>

make: *** [Makefile:7: all] Error 1

Issues/PRs references

Fixes #20322 (comment)

[maribu]

@MrKevinWeiss Do you want a backport? I think the chances of breaking stuff are pretty low with this one :)

[riot-ci]

Murdock results

✔️ PASSED

8ea106c build system: add tl;dr to error message

Success	Failures	Total	Runtime
1	0	1	54s

Artifacts

• Documentation preview

[benpicco]

Only minor changes indeed, nothing controversial.

[MrKevinWeiss]

@MrKevinWeiss Do you want a backport? I think the chances of breaking stuff are pretty low with this one :)

It is nice but I don't think critical for this release.