Start index files with H1

[wd60622]

Let's start the index.md files with an H1 tag to avoid the warnings building documentation builds.

We'd want to check to see if this switch results in rending of the documentation, like an inclusion of title in different location of toc, etc.

Here are the warnings that we are getting:

pymc-marketing/docs/source/getting_started/installation/index.md:1: WARNING: Document headings start at H2, not H1 [myst.header]
pymc-marketing/docs/source/getting_started/installation/index.md:39: WARNING: Document headings start at H2, not H1 [myst.header]
pymc-marketing/docs/source/index.md:39: WARNING: Document headings start at H2, not H1 [myst.header]
pymc-marketing/docs/source/index.md:56: WARNING: Document headings start at H2, not H1 [myst.header]
pymc-marketing/docs/source/index.md:111: WARNING: Document headings start at H2, not H1 [myst.header]
pymc-marketing/docs/source/index.md:130: WARNING: Document headings start at H2, not H1 [myst.header]