docs: Merge xsd-fu prerequisites into build instructions

Merge source code page with build instructions
Move build instructions to the top since it's got a wider
scope than contributing--non-developers need this
merge dev_5_0 -D org: PR 1196 (Close IFormatReader copy if not to be used)
merge dev_5_0 -D org: PR 1197 (Add modulo support for LI-FLIM data (rebased onto dev_5_0))
merge dev_5_0 -D org: PR 1198 (ScanR: fix exposure time units (rebased onto dev_5_0))
merge dev_5_0 -D org: PR 1201 (Leica LIF: do not set Channel.Color for RGB images (rebased onto dev_5_0))
merge dev_5_0 -D org: PR 1202 (DICOM: fix units for physical sizes (rebased onto dev_5_0))
merge dev_5_0 -D org: PR 1207 (docs: Merge xsd-fu prerequisites into build instructions)
merge dev_5_0 -D org

Repository: openmicroscopy/bioformats
Excluded PRs:
  # PR 1206 sbesson 'Fix bfGetReader for empty input' (status: pending)
Already up-to-date.

Merged PRs:
  # PR 1196 joshmoore 'Close IFormatReader copy if not to be used'
  # PR 1197 melissalinkert 'Add modulo support for LI-FLIM data (rebased onto dev_5_0)'
  # PR 1198 melissalinkert 'ScanR: fix exposure time units (rebased onto dev_5_0)'
  # PR 1201 melissalinkert 'Leica LIF: do not set Channel.Color for RGB images (rebased onto dev_5_0)'
  # PR 1202 melissalinkert 'DICOM: fix units for physical sizes (rebased onto dev_5_0)'
  # PR 1207 rleigh-dundee 'docs: Merge xsd-fu prerequisites into build instructions'
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