Update docs license

[yasahi-hpc]

Resolves #86
Resolves #84

In this PR,

1. Update the License to The Apache 2.0 License with LLVM Exception or the MIT License.
2. Update the paths in README.md and docs
3. Embed new licenses

[jbigot]

We should offer both MIT & Apache-2 at the user's discretion

[jbigot]

Also, we should follow reuse spec

[yasahi-hpc]

@jbigot Modified accordingly

[jbigot]

I have added reuse compliant licences, as well as a check, but now the line length check does not pass anymore, I must have a look at that.

[jbigot]

Fixed, should be good now

[yasahi-hpc]

@jbigot
Thanks!
I have two questions.

1. Under LICENSES folder, we have BSD and CC0 licenses. Do we need them?
2. It seems that .txt files are just templates. We do not need to embed our information?

[jbigot]

We need BSD & CC0 because we have a few files that use them, namely FindFFTW for BSD & the reuse check for CC0, if we remove them the check will fail.

I don't believe we should embed any info in the licence files. The info is available from other places.

[yasahi-hpc]

Sure. Thank you for your explanation.
I will merge this and work on other issues