Add new landing page CSS to this repo

## Overview

In https://github.com/elastic/kibana/pull/138051 @gchaps created a new landing page for the Kibana docs. There's some interest in using a similar layout for other docs (for example, https://github.com/elastic/observability-docs/issues/2209).

Most of the styling for the page uses either (1) Bootstrap CSS (see https://github.com/gchaps/kibana/pull/1) or (2) global styles defined in [this repo](https://github.com/elastic/docs/tree/master/resources/web/style). However, there are a few custom style classes needed to structure the page. Currently those classes are included [in the HTML directly inside `` tags](https://github.com/elastic/kibana/blob/ae43e594ca844a23955273b7a36b7e8c143370cf/docs/index-custom-title-page.html#L1-L48).

We should move these landing page specific styles to this repo to make it easier to maintain consistency _especially_ if we plan to build similar pages for other books. But, I'm hesitant to move those styles to this repo before publishing this landing page because of the issues we saw with caching of front-end assets that caused many users to be served a mix of old and new HTML/CSS/JS when we published https://github.com/elastic/docs/pull/2478 (for example, https://github.com/elastic/docs/issues/2510). I think we should figure out a cache-busting strategy for front-end assets (perhaps https://github.com/elastic/docs/issues/2509) before moving the landing page styles to this repo.

In the short term, @ryankeairns @gchaps and I decided it would be ok to publish https://github.com/elastic/kibana/pull/138051 with the custom styles included in the HTML in the /kibana repo. I _think_ this would help us avoid the caching issues because the styles will be in a _new_ file (rather than changes in an existing repo).

## Next steps

- [ ] Publish https://github.com/elastic/kibana/pull/138051 with styles included in the HTML
- [ ] Figure out a cache-busting strategy for front-end assets
- [ ] Add landing page styles to /docs
- [ ] Remove landing page styles from /kibana
- [ ] Other teams build landing pages using global styles as needed

cc @ryankeairns @gchaps @gtback @bmorelli25