Fix incorrect handling for the summary key in documentation metadata

[kuanchihwang]

When the summary key is used in documentation metadata, FORD generates garbled contents for the summary field of the documented program unit.

For example, with the documentation metadata below,

!> summary: Lorem ipsum dolor sit amet, consectetur adipiscing elit.
!>          Fusce ultrices tortor et felis tempus vehicula.
!>          Nulla gravida, magna ut pharetra.

FORD generates the following HTML source code, which is obviously incorrect:

<p>L\no\nr\ne\nm</p>\n<p>i\np\ns\nu\nm</p>\n<p>d\no\nl\no\nr</p>\n<p>s\ni\nt</p>\n<p>a\nm\ne\nt\n,</p>\n<p>c\no\nn\ns\ne\nc\nt\ne\nt\nu\nr</p>\n<p>a\nd\ni\np\ni\ns\nc\ni\nn\ng</p>\n<p>e\nl\ni\nt\n.</p>\n<p>F\nu\ns\nc\ne</p>\n<p>u\nl\nt\nr\ni\nc\ne\ns</p>\n<p>t\no\nr\nt\no\nr</p>\n<p>e\nt</p>\n<p>f\ne\nl\ni\ns</p>\n<p>t\ne\nm\np\nu\ns</p>\n<p>v\ne\nh\ni\nc\nu\nl\na\n.</p>\n<p>N\nu\nl\nl\na</p>\n<p>g\nr\na\nv\ni\nd\na\n,</p>\n<p>m\na\ng\nn\na</p>\n<p>u\nt</p>\n<p>p\nh\na\nr\ne\nt\nr\na\n.</p>

The expected output should be:

<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce ultrices tortor et felis tempus vehicula. Nulla gravida, magna ut pharetra.</p>

This PR ensures that the summary field correctly contains a single paragraph of text.

---

To reproduce this issue, consider the minimal reproducible example below.

1. test_summary_key.md

---
project: Test Summary Key
summary: Lorem ipsum dolor sit amet, consectetur adipiscing elit.
         Fusce ultrices tortor et felis tempus vehicula.
         Nulla gravida, magna ut pharetra.
author: Tester
---

Test the `summary` key in project and documentation metadata.

2. src/test_summary_key.f90

!> summary: Lorem ipsum dolor sit amet, consectetur adipiscing elit.
!>          Fusce ultrices tortor et felis tempus vehicula.
!>          Nulla gravida, magna ut pharetra.
!>
!> Descriptions for module `m`.
module m
    implicit none
contains
    !> summary: Lorem ipsum dolor sit amet, consectetur adipiscing elit.
    !>          Fusce ultrices tortor et felis tempus vehicula.
    !>          Nulla gravida, magna ut pharetra.
    !>
    !> Descriptions for function `f`.
    function f(a, b)
        integer, intent(in) :: a, b
        integer :: f

        f = 0
    end function f

    !> summary: Lorem ipsum dolor sit amet, consectetur adipiscing elit.
    !>          Fusce ultrices tortor et felis tempus vehicula.
    !>          Nulla gravida, magna ut pharetra.
    !>
    !> Descriptions for subroutine `s`.
    subroutine s(a, b)
        integer, intent(in) :: a, b
    end subroutine s
end module m

3. Run FORD to generate the documentation. Inspect the "Modules" and "Procedures" pages.

[ZedThree]

Thanks @kuanchihwang! Please could you add a test in test_sourceform.py? You can copy test_markdown_codeblocks_bug286 and test_markdown_header_bug286 for how to write this.

[ZedThree]

Thanks @kuanchihwang!