Add katex support in documentation.

[mmaker]

This commit adds latex support within rustdoc for arkworks algebra.
In order to add latex code in textmode, use delimiters $ and $.
In order to add latex code in displaymode, use delimiters $$ and $$.
To compile the documentation, use cargo rustdoc.

This is achieved via the option --html-in-header set via

• the environment variable RUSTDOCFLAGS, or
• the variable build.rustdocflags of .cargo/config.toml
  I opted for the latter making it a default compilation flag to use with
  rustdoc.

IMPORTANT: for some reason (independent from us) cargo rustdoc fails
on rust nightly and this is due to a failure concerning cfg-if and
rand-chacha. I did not investigate the reason why this happened.

[mmaker]

Great, right now doctests (cargo test) fail because the documentation path is incorrect:
instead of doc/katex-header.html it expects ../doc/katex-header.html.
However, cargo rustdoc expects doc/katex-header.html instead of ../doc/katex-header.html.

We have two options:

• we YOLO it, and let the user set the option --html-in-header or the env variable RUSTDOCFLAGS manually.
• we create a makefile inside each package with the following content:

doc:
	cargo rustdoc -- --html-in-header doc/katex-header.html

The latter is what's being used in curve25519-dalek, but would pollute the build environment :(
This affects only local compilation and shouldn't concern upload to docs.rs, which is already dealt with.

[Pratyush]

@mmaker would using the solution in this issue work? rust-lang/cargo#9427

We would have to switch to running doctests on nightly only, but that's fine IMO

[mmaker]

After some investigation:

rustdocflags = "-Zdoctest-in-workspace --html-in-header doc/katex-header.html"

clearly does not work because it's not a rustdoc option. After some hacking around, the following option is what we want:

[unstable]
doctest-in-workspace = true

Now .cargo/config.html looks like the following:

# Add support for katex, and work around rust-lang issue #8097.
# <https://github.com/rust-lang/cargo/issues/8097>
[target.'cfg(nightly)']
rustdocflags = "--html-in-header doc/katex-header.html"

# Without this line, one among `cargo test --doc` and `cargo rustdoc`
# will fail due to the relative path in the `--html-in-header` option for rustdoc.
[unstable]
doctest-in-workspace = true

Note the target.cfg(nightly): without it, on rust stable we'd have cargo test --doc fail because of the relative path issue...
However, now we have nother problem: target.<cfg> sections only define rustflags and not rustdocflags... therefore we're back to square one.

[mmaker]

Force-pushing this pull request with a makefile-based workaround, similarly to what's done in curve25519-dalek...