ci: yarn install before typedoc

[turadg]

Description

The latest failure (https://github.com/endojs/endo/actions/runs/7107980418/job/19350413938) is that the imports don't resolve:

Run npx typedoc
npm WARN exec The following package was not found and will be installed: typedoc@0.25.4
[info] Converting project at ./packages/base[6](https://github.com/endojs/endo/actions/runs/7107980418/job/19350413938#step:4:7)4
[warning] No entry points were provided, this is likely a misconfiguration.
[info] Converting project at ./packages/bundle-source
[info] Converting project at ./packages/captp
packages/captp/src/atomics.js:1:23 - error TS26[8](https://github.com/endojs/endo/actions/runs/7107980418/job/19350413938#step:4:9)8: Cannot find type definition file for 'ses'.

1 /// <reference types="ses"/>

That's because there was no yarn install. So this does that. No more npx.

Testing Considerations

I did a fresh checkout of the repo and ran npx typedoc to confirm the hypothesis. Then I ran yarn install && yarn docs and that worked.

EDIT: and I discovered that workflow_dispatch lets you pick a branch to load and run a workflow. https://github.com/endojs/endo/actions/runs/7108099837/job/19350750851 shows the site building now, just not deploying because of branch permissions.