Yes, C=C symbolizes a double bond between two carbon atoms. Benzene, which has 6 carbons in a connected in a ring, is often drawn with alternating single and double bonds between all the carbons in the ring, such as this, but with the ends connected from the first carbon to the last, -C=C-C=C-C=C-

This is called "conjugation". It represents more than just a simple double bond between adjacent atoms - it denotes that electrons are being shared among the atoms, and the electrons are then free to "run around the ring", so to speak, and not be trapped inside a bond between specific adjacent atoms. Another way this is sometimes drawn is to draw all the carbons in the ring as having single bonds between them, and then on the inside of the ring, a dashed line is drawn all the way around. The dashed-line represents the conjugation - the sharing of the electrons from the double bonds with the single bonds. Some people like to think of these bonds as having 1.5 electrons in them.

Most certainly benzene, hydroquinone, and catechol have carbon-carbon double bonds (C=C), not only that, they are conjugated double bonds. They are by definition aromatic compounds, all sharing this property of having alternating carbon-carbon double and single bonds in a ring stucture.

Keep in mind this conjugation can extend into multiple rings, or even conjugated carbons connected to a ring, or multiple rings, and then more conjugated carbons. Or the reverse where there is an aromatic group connected with a string of conjugated carbons and then another aromatic group is on the opposite end. This structure is often used in photographic sensitizing dyes. Check out the structure of some organic dyes with these propterties. And keep in ming that nitrogens and oxygens can be incorporated into the conjugation.

Here's a link to cyanine dyes as used in photography:

And it's the conjugation in these developer compounds that allow them to spread the charge of the electon about the surface of the developer molecule that is at the heart of the subject here with the Kendal-Pelz rule.

See these pages for more info, if you like: