Form cleanup/redirects (enterprise trial -> new demo page)

[linear[bot]]

Since we're not really doing enterprise trials (as far as I know), let's 1. clean up the messaging/copy of the most common links pointing to it and instead point to the new demo page and then 2. redirect the enterprise trial page to the new demo page.

Top sources / Locations on website:

• bottom of code search page (done!) -biggest source by far
• https://sourcegraph.com/docs/admin/deploy
• https://sourcegraph.com/docs/cloud
• https://sourcegraph.com/enterprise-trial-offer?form_submission_source=cody-enterprise-trial