[DO NOT MERGE] Origin trials docs

[alexandrascript]

Move the origin trial concept docs into the OT section.

[chrome-devrel-infra[bot]]

:x: Failed to build (static) the site for commit 7af5424c5a82a8b63398ff8ecc3dc8327181062b. Building the site (npm run production) failed with the following error output:

[36m
[1msite/_js/main.js, site/_js/100.js, site/_js/events.js, site/_js/styleguide.js, site/_js/fugu-showcase.js, site/_js/extension-samples.js[22m → [1mdist/js[22m...[39m
[32mcreated [1mdist/js[22m in [1m2s[22m[39m
[11ty] 1. Having trouble parsing data file ./site/_data/docs/web-platform/toc.yml (via TemplateDataParseError)
[11ty] 2. bad indentation of a mapping entry (25:14)
[11ty] 
[11ty]  22 | - title: i18n.docs.web-platform.origin-trials
[11ty]  23 |   sections:
[11ty]  24 |     - title: i18n.docs.web-platform.ot-concepts
[11ty]  25 |         - url: /docs/web-platform/origin-trials
[11ty] -------------------^
[11ty]  26 |         - url: /docs/web-platform/third-part ...
[11ty]  27 |         - url: /docs/web-platform/origin-tri ... (via YAMLException)
[11ty] 
[11ty] Original error stack trace: YAMLException: bad indentation of a mapping entry (25:14)
[11ty] 
[11ty]  22 | - title: i18n.docs.web-platform.origin-trials
[11ty]  23 |   sections:
[11ty]  24 |     - title: i18n.docs.web-platform.ot-concepts
[11ty]  25 |         - url: /docs/web-platform/origin-trials
[11ty] -------------------^
[11ty]  26 |         - url: /docs/web-platform/third-part ...
[11ty]  27 |         - url: /docs/web-platform/origin-tri ...
[11ty]     at generateError (/workspace/node_modules/js-yaml/lib/loader.js:183:10)
[11ty]     at throwError (/workspace/node_modules/js-yaml/lib/loader.js:187:9)
[11ty]     at readBlockMapping (/workspace/node_modules/js-yaml/lib/loader.js:1182:7)
[11ty]     at composeNode (/workspace/node_modules/js-yaml/lib/loader.js:1441:12)
[11ty]     at readBlockSequence (/workspace/node_modules/js-yaml/lib/loader.js:1008:5)
[11ty]     at composeNode (/workspace/node_modules/js-yaml/lib/loader.js:1440:12)
[11ty]     at readBlockMapping (/workspace/node_modules/js-yaml/lib/loader.js:1164:11)
[11ty]     at composeNode (/workspace/node_modules/js-yaml/lib/loader.js:1441:12)
[11ty]     at readBlockSequence (/workspace/node_modules/js-yaml/lib/loader.js:1008:5)
[11ty]     at composeNode (/workspace/node_modules/js-yaml/lib/loader.js:1440:12)

Check the Stage (dcc-staging) check status for more details.