compress the storage size of Interpolations.jl

It's pretty big a downloading size for Interpolations from pkg server and I think it's better to optimize this:

```
# Interpolations@v0.12.10
$ wget https://pkg.julialang.org/package/a98d9a8b-a2ab-59e6-89dd-64a1c18fca59/2b7d4e9be8b74f03115e64cf36ed2f48ae83d946

$ du -sh 2b7d4e9be8b74f03115e64cf36ed2f48ae83d946
4.6M 2b7d4e9be8b74f03115e64cf36ed2f48ae83d946
```

As a comparison, `Images@0.22.3` has only 60K size.

```julia
#Images@v0.22.3
$ wget https://pkg.julialang.org/package/916415d5-f1e6-5110-898d-aaa5f9f070e0/86f30ea4fe483a3a4c99b2e0ae2065fb006d345e

$ du -sh 86f30ea4fe483a3a4c99b2e0ae2065fb006d345e
60K 86f30ea4fe483a3a4c99b2e0ae2065fb006d345e
```

---

A major part of them are contributed by the documentation:

```julia
$ du -sh ~/Documents/Julia/Interpolations.jl

13M doc
1.1M docs
4.0K LICENSE.md
4.0K NEWS.md
400K perf
4.0K Project.toml
8.0K README.md
152K src
172K test
20K test.svg

$ du -sh ~/Documents/Julia/Interpolations.jl/doc

4.0K doc/Extrapolation.md
56K doc/images
12K doc/Interpolations.jl.ipynb
48K doc/Interpolations_algebra.nb
160K doc/Interpolations_algebra.pdf
8.0K doc/Math.md
13M doc/Plotting examples.ipynb
```