frontmatter(banners): Rename `seecompattable` value to `experimental`

[OnkarRuikar]

• related to feat(frontmatter): support banners #260

1. We haven't updated the content yet. So it is the right time to rename the seecompattable value to experimental_header.
2. If the frontmatter key is called banners, then why do the values need a redundant _header suffix? Shouldn't it be like:

   banners:
     - securecontext
     - experimental
     - deprecated
     - non-standard

3. The platform code calls it banner, so why should the content side call it header? Can we have the same name everywhere?