Add Bazel build support

[Vertexwahn]

This PR helps users of the Bazel build system to integrate JSON for Modern C++ in their builds.

[gregmarr]

See PR #1606 for a previous attempt that has questions that apply to this PR as well.

[coveralls]

Coverage remained the same at 100.0% when pulling 21f4fbb on Vertexwahn:add-bazel-support into 307c053 on nlohmann:develop.

[nlohmann]

Yes, please add documentation and check the linked PR.

[nlohmann]

• Please add documentation (see https://json.nlohmann.me/integration/package_managers/ or https://json.nlohmann.me/integration/cmake/).
• Please check the discussion at add bazel build #1606.

[falbrechtskirchinger]

I have a very clear picture of how this should work and how it should integrate with generating release artifacts, etc. This isn't it.
Specifically, I'd prefer a CMake script (i.e., a CMake file that can be invoked via cmake -P) that is tailored to our needs instead of adding Google's generalized CMake functions. Also, there's no need for a CMake option and generating the file during the build is only useful if we're going to install it. (Do we want that? What's the proper install directory? Can BUILD.bazel be made relocatable or do we have to hard-code absolute paths?)
We can decide later if we want to add the script to the amalgamation process and always keep it up-to-date or only update it manually. (I'd start with the latter and move to the former after the amalgamation stuff has been cleaned up.)

So here's my suggestion @Vertexwahn: If you finish the documentation as requested by Niels here (the relevant files are in docs/mkdocs/docs/integration/), I'll work on the CMake code for this PR (shouldn't require more than a handful of lines).

[Vertexwahn]

@falbrechtskirchinger I added a Bazel paragraph to the Integration section. It would be nice if you could work on the CMake code. BTW: The Bazel files need not be installed. I can maintain the manual update of the Bazel files at least for the next 6 months.

[Vertexwahn]

• Please add documentation (see https://json.nlohmann.me/integration/package_managers/ or https://json.nlohmann.me/integration/cmake/).
• Please check the discussion at add bazel build #1606.

I added to the README as well as the integration documentation a section about Bazel

[falbrechtskirchinger]

@falbrechtskirchinger I added a Bazel paragraph to the Integration section. It would be nice if you could work on the CMake code. BTW: The Bazel files need not be installed. I can maintain the manual update of the Bazel files at least for the next 6 months.

I've submitted a PR to your fork with the relevant changes. Please merge and we can continue here.

[falbrechtskirchinger]

Two minor whitespace issues.
Otherwise, looks good to me.

[Vertexwahn]

@nlohmann Can it be merged now or is there any further action needed?

[falbrechtskirchinger]

@Vertexwahn He's on vacation. I'm sure this will be merged shortly after he gets back.

[nlohmann]

Thanks!