Improve documentation page for plane curves

[chriswuthrich]

as the title says, the index of plane curves is a long list, which we should split up so that it is easier to navigate.

Depends on #16806

CC: @JohnCremona

Component: elliptic curves

Author: Chris Wuthrich

Branch/Commit: 83b67ff

Reviewer: John Cremona

Issue created by migration from https://trac.sagemath.org/ticket/17006

[chriswuthrich]

Author: Chris Wuthrich

[chriswuthrich]

Branch: u/wuthrich/ticket/17006

[chriswuthrich]

Dependencies: #16806

[chriswuthrich]

Last 10 new commits:

4b1a2d4	Revert "Revert "Updated Sage version to 6.3""
12c6f01	comitting because git wants me to?
dc0185c	sorted out now?
8771832	forgot to import Set
cadd58f	Merge remote-tracking branch 'trac/u/elarson3/isogeny_bounds_for_elliptic_curves_over_number_fields' into larson
828ed6f	#16806: reviewer's patch
0f4e30b	Merge branch 'develop' into larson
be81f80	#16806: minor changes after review
8c93016	trac 16806: include in documentation
83b67ff	trac 17006: improve documentations for plane curves

[chriswuthrich]

Commit: 83b67ff

[chriswuthrich]

comment:2

I wrote it on top of #16806 to avoid merging problems.

I really only restructured the index of plane_curves. Included one more file lseries_ell and made a few cosmetic changes.

Of course, it is not perfect, but it is an improvement and for now that is what I am willing to put in as time to improve it.

[JohnCremona]

comment:3

It builds fine and looks good. Thanks!

[JohnCremona]

Reviewer: John Cremona

[vbraun]

Changed branch from u/wuthrich/ticket/17006 to 83b67ff