WIP: Add note on how to estimate appropriate values for alpha

[jarrodmillman]

Fixes #2307.

I made parallel changes to both katz_centrality and katz_centrality_numpy. In particular, I added the sentence

    You can use ``max(nx.adjacency_spectrum(G))`` to get $\lambda_{\max}$ the largest
    eigenvalue of the adjacency matrix.

after the sentence (in katz_centrality)

    The parameter ``alpha`` should be strictly less than the inverse of largest
    eigenvalue of the adjacency matrix for the algorithm to converge.

and the sentence (in katz_centrality_numpy)

    The parameter ``alpha`` should be strictly less than the inverse of largest
    eigenvalue of the adjacency matrix for there to be a solution.

You will also notice that I changed the inline math markup (e.g., :math:`\beta` became $\beta$). This uses the texext Sphinx extension, which I added in PR #2588. I find the dollar_math much more readable and familiar when I am working with source files and it renders the same in HTML.