Add documentation for Page draft

[dadarya0]

This PR adds documentation for the Page Draft feature found in mautic/mautic#13937

[escopecz]

@RCheesley I'm not sure how to deal with this Vale warning

Is this referring to a Mautic feature? If so, use 'Landing Page' instead of 'page'.

It should be lower case p. Do we have to ignore the Vale check on this line?

[RCheesley]

If you're referring to a Mautic Landing Page then both words are capitalised.

[RCheesley]

Hi folks,

A few things which I've fixed in this PR so we can merge it:

1. You must always ensure that the headings are the same underline length as the text above them - if you run the command make html in your /docs folder it will flag up such warnings to you, tab complete when you start typing if you use VSCode usually does it right but sometimes misses the odd character!
2. I think the heading should be H2 instead of H3 so I changed those.
3. There was some inconsistency with draft v Draft, I've matched the UI with using the capitalised version
4. An Landing Page Draft allows changes to the content of the Landing Page only. > changed to active voice: You can only change the content of the Landing Page itself when it's in Draft.
5. An Landing Page Draft may be previewed by appending > changed to active voice: You can preview a Landing Page Draft by appending
6. a Draft Preview URL will be > a Draft Preview URL is
7. There was a typo on line 49 which I fixed (val on rather than vale on) which was hiding some of the later errors.