Small doc fixes (#499)

* A small shift in notation

* Docs: Clarify ex2-surface mathematical description

* Refactor \boldsymbol -> \bm

* Refactor \mathbf -> \bm

* Sphinx: clear html_stati

Small doc fixes (#499)

* A small shift in notation

* Docs: Clarify ex2-surface mathematical description

* Refactor \boldsymbol -> \bm

* Refactor \mathbf -> \bm

* Sphinx: clear html_static_path

This is needed to avoid the 'html_static_path entry '_static' does not exist' warning

https://github.com/readthedocs/readthedocs.org/issues/1776

* Fix some incosistencies

* Invert inner products (u,v) -> (v,u)

* Add SVG figure for coordinates on the sphere

* doc: use dirhtml on readthedocs

* Improve caption of sphere coords figure

* Add reference to figure in the bps on the cubed-sphere section

Co-authored-by: Jed Brown <jed@jedbrown.org>

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