Adding logo assets

[portsmouth]

Adding here the finalized logo. This is almost identical to the one posted on Slack, except:

• extremely small adjustment to the arrow head, to make it perfectly centered on and orthogonal to the stroke

• SVG was optimized to remove redundant elements (e.g. gradients). Main logo is now only 10Kb. PNGs exported at 800dpi.

Assets

White paper

README

[portsmouth]

An admin is needed to change the project icon to the OpenPBR glyph.

[portsmouth]

Some alternative layouts for the title and logo placement.

I like the second version the most, with the logo+text used for the title, and the subtitle giving the detailed "OpenPBR Surface specification" and the version/date.

**OpenPBR Surface**
<center>*Specification v1.1, 2024-06-28*.</center>

<center>
   <a href="https://github.com/AcademySoftwareFoundation/OpenPBR" title="Redirect to GitHub page"><img src="images/logo/openpbr-logo.svg" alt="homepage" height="48" style="vertical-align: middle; position: relative; bottom: 0px"/></a>
   &nbsp;&nbsp;   &nbsp;&nbsp;
   <a href="https://www.aswf.io/" title="Redirect to ASWF page"><img src="images/aswf-logo-black.svg" alt="homepage" height="36" style="vertical-align: middle; position: relative; bottom: 0px"/></a>
</center>

<center>
   <a href="https://github.com/AcademySoftwareFoundation/OpenPBR"><img src="images/logo/openpbr-logo-glyph.svg" alt="homepage" height="80" style="vertical-align: middle; position: relative; bottom: 0px"/></a>
   &nbsp;
   <a href="https://github.com/AcademySoftwareFoundation/OpenPBR"><img src="images/logo/openpbr-logo-text.svg" alt="homepage" height="100" style="vertical-align: middle; position: relative; bottom: 0px"/></a>
</center>

<style>bigger { font-style: italic; font-weight: normal; font-size: 115%;}</style>
<center><bigger><i>OpenPBR Surface specification v1.1, 2024-06-28</i></bigger></center>

[portsmouth]

In aea298251776e719e9da6fd0801cf79cec59a70c added simplified forms of the logo suitable for icons, as requested/discussed on Slack.

[portsmouth]

In 54474ff8268725f84b852e027c29e7b755326beb updated the spec to use the logo (glyph next to text form) for the title, with the subtitle giving the full "OpenPBR Surface" (+version/date) wording.

[portsmouth]

Added proposed 128x128 favicon in 95cc4baf477684eeedcf3f5657078b861b58271e

[meshula]

looking good!