Add documentation for hefquin-rmlmat and for hefquin-client

[KentThang]

Closes #654.

The page feels a little small right now and follows the same structure as cli.html. Should I perhaps add a brief description of how it works or is it sufficient as is?

[hartig]

I edited the new documentation page myself (see the new commit in this PR). In particular, I added an initial paragraph that provides some context and I changed the rest of the text a bit.

However, there is still an open task for you.

[hartig]

When I just edited your text on this page here a bit, I was wondering whether it shouldn't be possible to also specify the serialization format of the output. Now, when I do

hefquin-rmlmat --help

I see more arguments than the ones listed here, including arguments related to serialization format. While you do not need to include all of the arguments of --help on this page, it would be nice if you could add the ones that show up under 'Output control'.

[hartig]

Regarding the last sentence, this doesn't seem right. Except for N-Triples files, it is not really possible to append to an RDF file. I have fixed this issue in PR #670. The corresponding change to this part of the documentation is:

Suggested change

	Use this argument if you want the produced RDF data to be written into a file. If this argument is omitted, the result is written to stdout. If the given file already exists, output is appended rather than overwritten.
	Use this argument if you want the produced RDF data to be written into a file. If this argument is omitted, the result is written to stdout. If the given file already exists, it will be overwritten.