topic: tutorial grammar fixes

doc-front-page: link to the changeset evolution section in the document The section is unlikely to be perfect, but at least it exists.

doc-front-page: use a clearer statement about the experimental status This text comes form the Mercurial wiki.

doc-front-pagE: add a paragraph about "why the extension" Let us clarify why there is an extension in the first place.

doc-front-page: expand the paragraph about changeset evolution We clarify the part of the explanation about the changeset evolution concept.

doc-front-page: rename the "next step" section This are pointer to relevant section in the doc. It is a bit double usage with the table of content. We at least clarify the title.

doc-front-page: mention that "Changeset Evolution" is a specific concept It will get it own paragraph soon.

doc-front-page: include the index in the TOC This will help important content to stand forward.

doc-front-page: add a title to the part about the extension We'll rework this section more in depth, but I would like it to have a title first to have it show up in the table of content.

doc-front-page: add a feature summary at the beginning of the page